I don't know who you spoke to, but not only is the Amp capable of driving 2 pairs of the Archtectural speakers, it can drive 3 pairs.  You are correct though, Trueplay will not work for mismatched models.  Trueplay relies on the timbre being exactly the same for all speakers connected to the Amp. 

https://support.sonos.com/en-us/article/connect-six-sonos-architectural-speakers-to-amp

You could temporarily disconnect the 3rd party speakers and run Trueplay. I have no idea how effective this will be in terms of “best” overall sound for the room or if AMP will discover your trick and ignore the Trueplay adjustments after the 3rd party speakers are reconnected.

Since you're not afraid of discussions with Sonos tech personnel: would it be an idea to call them about this? Since in-walls and in-ceilings should be Truplay capable (please correct me if I'm wrong), maybe someone can explain why you would get ““unable to complete to architectural speaker detection” error.’

The bolded part needs clarification.  Using two Sonos Ones as surrounds implies you’re setting up a HT room connected to a TV.  You will an Arc, Beam, Ray, or Amp for the front audio channels and to connect to your TV.  Is your intention to use the Amp currently powering your 4 in ceiling/wall speakers  as the front audio channels?  Is the TV in your loft or in your MBR?

Maybe I’m not visualizing your setup correctly, but this sounds like you’re not going to have speakers located in the correct/normal places  and trueplay tuning would be the least concerning thing about the setup.

But you also stated that your amp driven speakers are just for music, so I don’t know.

Thanks everyone so far for the feedback, sincerely appreciate it.  Additional clarifications are outlined below.

The speakers in the both the loft and MBR will only be used for music. There is no television in either rooms.  The rooms are tv free zones.   The goal is to create surround sound with the Sonos Architectural speakers, the Sonos Sub and 2 Ones.  Since the loft and MBR are separated by a flight of stairs and the loft overlooks the master bedroom, I would like to connect the speakers and the loft as one room versus two. (Prior set up was split into two rooms, with Play 5s in the loft.)  

After the installation was done and I received the diagnostic “unable to complete architectural speaker detection,” I immediately contacted Sonos Technical Support and was advised by Sonos tech support that Trueplay should work with the current set up and that I did not need a second Amp.  I pointed out to Sonos the link below and then they confirmed it would not work and recommended that I return one of the speakers.  Problem is, the speakers are already installed and thus to recommend that I return  a pair of the speakers does not make any sense.  It appears the only option is to purchase another Amp which I am not a happy camper about.   

https://support.sonos.com/en-us/article/unable-to-complete-architectural-speaker-detection

I think I follow now.  Your actually setting up a Home Theatre setup without actually connecting to a TV.  The Amp is your “master” that would normally be connected to the TV.  You’ll configure the Ones as surrounds set to play “full’ so they play stereo for music, rather than muted. 

I don’t think it’s very accurate to refer to that as ‘surround sound’ since you’ve only got stereo audio, just with multiple speakers (6) covering the left and right channels, and they are not in the traditional locations for a home theatre setup.  Since trueplay tuning is designed for speakers in a home theatre configuration, I would not expect trueplay to really improve the sound much.  For example, the amp is supposed to cover the front speakers, and it sounds like you have them in the front and back.

I don’t know that I would setup a room like this.  Yes, it’s nice to have all 6 speakers as one room, but you can’t control the volume between the speakers.  You can set the relative volume between Ones and your speakers powered by amp, but that’s it.  If you want it louder in the loft, but not the MBR for example, you can’t do that. 

I wouldn’t get an Amp right now.  See what you think of the sound without trueplay before making that decision.  For one thing, your idea of having it be all one single Sonos room isn’t possible with 2 amps, 2 ones and a sub.  You can do 2 amps + sub , or 1 amp + 2 ones + sub.  If you try and think trueplay has to be there, and you’re ok having multiple rooms, then get the 2nd amp.

BTW, since you are techincally setting up a home theatre even though you don’t have a TV, you can use 2 subs in the single room.  A normal stereo pair only allows one sub.

1. Multiple speakers is not “surround sound” it’s just multiple speakers
2. 2 pairs of unlike models of architectural speakers will work just fine on a single Amp.  You just can’t run Trueplay.  

If you paste together this “Bedroom” (MBR + Loft) as a surround room, it will not be very convenient to adjust the relative level of each set of speakers. Using two AMP’s or one AMP and in-wall Volume controls will be more convenient (and straight forward for others living in the home to operate). As a surround Room, you will not be able to individually mute either area.

Trueplay is not designed to deal with this sort of lash-up.

Thanks once again for all of the input.  I spoke with a manager in technical support and he confirmed the following for what I am trying to achieve:

Room 1:  Amp and In-Wall Speakers or In-Ceiling Speakers but not both.

Rome 2:  Amp and In-Wall Speakers or In-Ceiling Speakers, Sub and 2 Ones. This is similar to the set up in the Media Room except the Media Room has a TV, Amp, 2 In-Wall, Sub and 2 Ones.

I will group Rooms I and 2 when in use, that is, when playing music.

Sonos recommended solution: another Amp. I concur, since removing already installed speakers was not an option.  Sonos promised to get back to me regarding obtaining the other Amp.  I will provide an update when I hear back from Sonos. 

Fortunately, one occupant, so inconvenience of others not a concern.  I have Sonos speakers throughout the home and I notice a difference with Trueplay.  

Thanks once again for the input from the Sonos Community.  I finally heard back from Sonos Technical Support on 01/16/23 and the issue was resolved to my satisfaction.   I received a one time, very reasonable courtesy price adjustment on the Amp.