Not sure what you mean by “front Sonos”.  Could you elaborate?

I am using for my TV: ARC SL, Five Left front, Five R front, Sub

I already have 2 in ceiling speakers and using the Sonos Amp S2 to power them, from a closet nearby, and hoped to join these 2 speakers to add the rear surround sound to the system; no luck...

It’s not possible to have a left front and right front with the Arc.  The Arc plays the left, center and right channels itself.  Is it possible you added the two Fives as surrounds?  If so, they are not placed correctly, they should be behind and to the sides of the main viewing area, not up front.  Or you could remove the Fives as surrounds and add the Amp so you can use the in-ceiling speakers as surrounds.  

Sorry, that doesn’t make any sense. The Sonos Arc already contains front right, center, and left speakers. 

The Sonos software doesn’t allow splitting the front right and front left signals to separate speakers. Is it possible that your Sonos Fives are already set up as surrounds? They’d show up in the controller as +LS (left surround) and +RS (right surround). 

If this is the case, you could remove these Fives from surround duty, and set them up as a separate ‘room’ in the Sonos software for music duty, and then bond the Amp as a surround driver, which would put those ceiling speakers in as surrounds. 

To be ultra clear, Sonos has no configuration in which any of their speakers can be a ‘center’ speaker. 
 

Edit: I type more slowly than @jgatie ;)

Thanks very much. I’m learning so much. It is really confusing. I hoped that I could attach my in ceiling speakers using my Amp to my Arc, instead of using the Fives, which I now learned were in the wrong position, but there is not logical space for them behind me, that's why I was hoping to utilize the in ceiling speakers.

If you remove/unbond the 2 x Fives from your Arc SL, you can ‘replace’ those with the Amp and the 2 ceiling speakers if you prefer …and then perhaps use the two Fives, either as separate Sonos rooms, or as a single stereo pair, which you can ‘group’ with any other rooms for music etc.

Thanks. Trying this now, and getting all sorts of errors. Going to try to reset and see if that help. Thanks so much for all your input.

I wouldn’t reset anything at this early stage - is it a new Sonos Amp, rather than the older Connect:Amp?

Here are the instructions for the newer Amp…

https://support.sonos.com/en-us/article/surround-sound-guidelines-and-limitations

If it’s the older Connect:Amp then see this link instead…

https://support.sonos.com/en-us/article/set-up-connect-amp-as-surround-speakers

The wording “all sorts of errors” does not help to help you.