Don’t attempt this. If you connect two amplifiers to a speaker the best outcome you can expect is that only one amplifier will blow up.

I have seen a wiring configuration before that goes in series through both the connect amp and a third party amp. Is this not doable? 

How did they eliminate the common ground?

If you want more power then a simpler, safer and neater solution would be to use whatever upgrade credit you have on the C;A to get a Sonos Amp.  Then sell the C:A on eBay (stating, of course, that the credit has been used).

I agree; note though that a doubling of amp power leads to an increase in sound levels of about 20%. 

At what levels on the volume slider of the Connect Amp are you having to run it at present to get the sound levels you need?

If power is a must, look to an amp with 200 wpc into 8 ohms or more. And that means a Sonos Port into a third party amp that has a sub level out. Make sure your neighbours and your Wharfedales can handle the consequences of letting that amp loose.

Thanks guys. 
so it seems that trying to wire the connect amp through another amp is a big no-no so I won’t be doing that. The guy I am buying the Yamaha also has a Connect so I will get that from him and either sell my connect amp or use it in another room with a nice pair of bookshelf speakers. Final question. Where to use the sub? 

Can you explain what the upgrade credit is? 

It’s not just the CONNECT:AMP, running any amplifier’s signal through another amplifier is not advised. 
 

The upgrade program, based on a simple Google search:

https://www.sonos.com/en-us/upgrade