2 cents from a newbee
I do not have back issues and only have 6 hours on my Silvio 2 (first recumbent bike I've ridden), but I can report that I have zero body pain from riding the Silvio. I can feel just 20 minutes on my well fitted DF bikes for several days after,... I "jumped" right onto the Silvio and spent an hour and felt no body issues. The next day I did an hour in the am and came back for an hour in the pm, no body pains. Repeated that the next day, no body pains. Back and neck feel well supported. If you can lay reclined on the couch you will probably be comfortable on the Silvio.
The riders head is up and looking out, ie good visibility.
There is no problem mounting the Silvio on a trainer.
There is a white knuckle (for me) learning curve that might cause issues with a bad back. Just from the wobbles, bobbles, and abrupt balance checks. And of course you both might be hyper-tuned into stressing-damaging your injured backs,...
I truly believe there might be some benefit to someone working up adult training wheels for the first 1-2 hours,... Really. It might have been helpfully to have a buddy hold the seat and get you moving for a little ways,...? I felt like I went right into the deep end, but I learned quick, and was "pedaling" in less than 10 minutes during my maiden voyage.
Good luck, Lavs