Not sure I understand your questions, but they seem to be related.

First, there is no Sonos Amp ‘for’ either indoor or outdoor, there is only the Sonos Amp. What you connect to it is your choice. 

It’s not clear from your question if you intend to ‘bond’ one of the Sonos Amps as surround speakers, but you can always ‘group’ Sonos products to play the same source. The only caveat is that when playing a TV input on the Arc, any ‘grouped’ room with that Arc will have a slight delay of around 75 ms at least. This normally isn’t an issue if you’re grouping an ‘room’ where the TV can’t be seen, the major issue is lip sync. Of course, if you can ‘hear’ both rooms, that might produce a slight echo/stadium effect, depending on where you stand. 

The design of Sonos is that all speakers (defined in Sonos software as a ‘room’) can be played to individually, or ‘grouped’ together. 

Hi. One Amp can be connected to the ceiling speakers in such a way as to work as rear surround speakers  Tnis is known as 'bonding. 

You can then group the otber Amp so that it plays the same audio as the HT setup.  For music this will sync perfectly. There will be a slight lag in the audio of the grouped Amp. You can probably adjust this away  but would not have perfect lip sync. 

Edit Bruce beat me to it :)

@pateldivyang84,

The Amp can be ‘bonded’ with a Sonos Arc for upto 5.1.2 Home Theatre audio. The Arc outputs the front L/C/R and upward firing (Atmos) channels and the connected Amp outputs the two rear L/R surround channels and the .1 LFE channel is merged into all, but up-to two Sonos Subs can be added to the setup for handling the Bass audio.

It is possible to quickly disconnect/reconnect the surrounds/Subs with a toggle switch in the Sonos App, but the Amps attached passive speakers then can’t be used separately unless you decide to unbond the Amp from the Arc. 

Unbonding/Rebonding devices from a HT setup can take longer to do and is not something you would perhaps want to do on any regular basis.

When bonded together, music audio can be played on the master HT device (Arc) with the bonded Amp surround-speakers output providing ambient support, although there is a switch in the App to play ‘full’ music audio to all speakers, albeit that switch does not have any effect when Atmos music audio is being played.

Personally speaking, I do not like ceiling speakers for HT front/rear channel audio and would only ever use such speakers for background music audio in perhaps a dining room,  just as an example. So I would use an amp for playing music only alongside the Arc and use other Sonos speakers, like Sonos Ones/OneSL’s at shoulder/head height for the two rear HT channels.

Note: The i/o ports on the Amp are also disabled when it is used for HT rear bonded surround speakers, so you can’t connect a 3rd party sub, or analog line-in audio source to it when used in that configuration.

If you want to use two sets of 6-8ohm speakers with a single Amp indoors and outdoors, I would opt for a switch, like the one mentioned in this YT video, but not use the Amp as part of the HT setup… but that’s just IMHO.

• https://youtu.be/2E0dYvihv_w

You can read more about the Sonos Amp at the links here:

• https://support.sonos.com/s/article/4717
• https://www.sonos.com/en-gb/shop/amp

Any separate Sonos Rooms can be grouped for music playback in perfect sync. 

However, with TV grouped room audio, there is a slight delay across grouped rooms compared to the video-synced output from the HT setup and if the grouped speakers are in the same physical room as the TV, then you will likely notice an echo in that room… usually fine though if the speakers are located in other rooms out of earshot of the main HT setup.

Not sure if I have covered all the questions you needed answering, but hope that information goes some way in assisting you in your purchase decision.