@Jwinter 

Im guessing you are using the EAP655 Wall Editions?  If so back to back isnt an issue, done that myself

In short, the wall edition has a signal pattern that is very directional with little to no bleed at that back (as its would literally be into the wall) so using these back to back is fine!

  @Philbert 

Thank you so much for your help.  I'm installing 6 room system. 5 upstairs each with guest and individual ssids. If back to backs have ssid guest, same frequency and same encryption password, that wont be an issue either? Still new at this including paranoid baby steps to not have to redo. 

  @Philbert 

Also going to IOT devices on another commonly Shared ssid. If all APs have common frequency, ssid and password,  you believe it still won't be an issue?

  @Jwinter 

It really depends on what you wish to achieve with these APs?     Are you wanting the APs to run as one network?   Ideally you should have each one on a seperate channel, especially if they are close enough to interfere, I would certainly recommend that.

Are you intending to use a controller to run these?  Ideally you should be!

  @Philbert 

Equipment acquired to date.

5 EAP655-wall

1 per each bedroom for wireless and wired connections for wall mounted tvs, laptops and gaming if desired.

All have Single (private shaired) ssid for permanent tenants 2 tenants. 

plus shaired Airbnb guest ssid for roamimg Airbnb guests. 

and iot shaired ssid for appliances.

and master ssid for daughter and I that also connects to last downstairs EAP655-wall for printer, TV and stereo 

if workable. = 4 ssids in all APs

2 EAP650 Outdoor for front patio and other for back yard garage wall for backyard property.

Each configured with (private, Airbnb guest, Iot and master ssids

= 4 ssids

2 EAP615 

1 in downstairs garage for iot and master ssid mostly 

1 inside backyard garage for same

4 ssids possibly 

TL-SG3428 Switch for cameras and APs.

MM Fiber to backyard garage Switch not purchased yet 

OC 200 controller purchased

  @Jwinter 

OK so you have a controller managing these, perfect that will solve your channel issues.  Just let the controller do the selection and you should be fine.   In terms of name and password, that doesnt affect it in way, its the signal strength and channel is the issue.

If you have a controller and SSID setup on the same channel, this is called a single channel architecture.. it will work fine as the whole channel is really one network in theory.. no issue with that.  However, its not really the accepted way to do it so you will likely find the controller will change the channels for you.    There are good and bad reasons for single channel vs multi channel setups, neither is perfect but the "more common" option is multi channel.

  @Philbert 

Thank you.  After I drill and install the APs I'm sure I will have more questions.  I appreciate your knowledge and experience.