Have to throw this out there..

@mark  My opinion says that if a thing is not disallowed by rule, regulation or law, it is allowed.

Mark that is not only officially specifically addressed by the FCC as permitted,  there are actual case histories of such installs.. 

I'll find it for you today if someone doesnt beat me to it (someone here probably has it on hand)

This isnt the document I had in mind but it still answers your question:

https://apps.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm?id=20973&switch=P

As long as the individual devices meet the specific Part 15 standard that applies for that frequency band and purpose, you can set up multiple transmitters.  I don't know how effective it would be (cost vs. return) but as long as all of Part 15 is followed, I don't see much of an issue.  

Radio Sausalito, based in Sausalito, CA has had a network of multiple transmitters (Rangemasters) for years now with, as far as I'm aware, no issues. Several of them are located to provide continuous coverage in one area, while two provide coverage in separate areas.

Of course, the fact that they've existed for years doesn't mean that it's allowed, but I believe that it is. Nice bit of detective work @richpowers, with finding that page on the FCC site!

Posted by: @rugster

↑

Radio Sausalito, ...Of course, the fact that they've existed for years doesn't mean that it's allowed, but I believe that it is. Nice bit of detective work @richpowers, with finding that page on the FCC site!

Well that page is almost as well known as the "200 foot". But it does include that peculiar phrase "non-coordinated transmitters" as being permitted, which we've talked about before, wondering what exactly it means... Does that then not imply that coordinated transmitters are not permitted?  Any implications are irrelevant, because it's not addressed anywhere in the rules, nor has it ever been. (Just as the 200ft document implications is irrelevant).

What's more, California's use of part 15 AM synchronized transmitters along the San Francisco Freeway was an ongoing project of which they worked directly with the FCC for about 3 years during the early 1970s. So that presidence alone is enough to confirm its legality, but there are other examples.

Also, keep in mind that the issue of multiple transmitters it refers to didn't have anything to do with part 15 AM, at least not directly. It applies to part 15 as a whole, but what actually brought it up was cordless phones and perimeter systems..

Excerpted from GEN. Docket No. 87-389
In the Matter of Revision of Part 15 of the Rules regarding the operation of radio frequency devices without an
individual license.
FIRST REPORT AND ORDER
Adopted: March 30, 1989;
Released: April 18, 1989

136. Use of Multiple Intentional Radiators. In the Notice, we proposed to prohibit the use of multiple radiators for the purpose of extending transmission range or for extending the area of coverage...

The comments in this proceeding generally object to this proposal. For example, AT&T states that it sees no need to exclude multiple emitters because the field strength limits will prevent significant interference from any given radiator and the authorization of such operation might enable the provision of a cellular-type building paging system under Part 15.

AT&T adds that multiple intentional radiators should be permitted in commercial environments when installed in accordance with the manufacturer's instructions, provided the field strength at a distance of 30 meters outside of the commercial property boundary does not exceed the limits in the proposed Section 15.109(c).

Sensormatic objects to the proposal since they use multiple antennas with their field disturbance sensors used for anti-theft tag sensors for the coverage of exits.

GM expresses similar concerns about applying this requirement to field disturbance sensors.

PA Consulting Group indicates that Part 15 multiple digital spread spectrum repeaters are desirable to permit high throughput coverage of an area.

Manufacturers of control and security alarm devices request the deletion of this provision to allow RE link relays for life-safety applications.

137. We are not adopting a prohibition on the use of multiple transmitters to extend transmission range or coverage area. We concur with the comments that multiple devices should be permitted provided the individual transmitters comply with the rules. However, we are denying the request by AT&T to allow multiple intentional radiators in a commercial environment with field strength limits measured 30 meters beyond the property boundary. The higher field strengths used by such systems would pose unacceptable potential for interference to authorized services.

This multiple transmitter rule  - or rather lack of any such rule, that the FCC addressed by confirming there is no rule, and in fact multiple transmitter use was already an established practice.

Well it kind of reminds me of how the FCC clarified the TIS rules a few years back to say that it was legal to air weather emergencies over TIS (How was that ever even a controversy?). So it made big news. However, soon after NOAA Weather Radio was designated in 1975 the sole Government-operated radio system to provide direct warnings of national disasters or nuclear attacks.  NOAA weather radios began getting really popular with the public.. and something that was big news was that TIS stations across the country were rebroadcasting daily NOAA weather reports!.. And then 40 years later the FCC writes a new rule that specifically allows it.

I should mention left out a paragraph out of the above document quotes. I left it out for the sake of truncuation, but realized it was inappropriate to omit as it seems to counter my point...

-------------------

136. Use of Multiple Intentional Radiators. In the Notice, we proposed to prohibit the use of multiple radiators for the purpose of extending transmission range or for extending the area of coverage.Biomedical telemetry devices were proposed to be excluded from this requirement as long as such devices were restricted to a localized area such as a hospital or residence...

(and later in the footnotes it says:)

Footnotes:
54. The definition of a biomedical telemetry device proposed in the Notice reflected the fact that these were the only devices proposed to be permitted to be employed as multiple transmitters. This distinction is no longer required, and the proposed definition is being changed accordingly.

----------------------

The a above clearly states that the original proposal would have allowed multiple devices only for biomedical telemetry devices, - but this was a new proposal, there had never been such a stipulation to part 15 rules before, and as we see, that proposal to restrict use of multiple devices to extend range never went through.

One other significant point is the predominant use of mesh networks almost everywhere today, which consist of an unlimited number of intentional part 15 transmitters in order to operate. Also of note, mesh networks can only function in a coordinated manner.- 

This observation is enough to confirm that there is no question that multiple part 15 transmitters is permitted and in common use, and that applies to transmitters which broadcast in the AM frequencies as well.

And yet another FCC confirmation that multiple part 15 transmitters may be used to cover a larger area. 

https://www.radioworld.com/news-and-business/headlines/fcc-wont-raise-unlicensed-power-levels-for-churches

“I understand that this makes it more challenging for churches during this difficult time, but I want to emphasize that there are options available,” Pai continued... Churches also could explore using multiple Part 15 devices to cover a larger area if they continue to provide ‘drive-in’ style services."