How to make reservation for myself and another passanger who’s not an interrail holder?

Hello all, 

I am planning to do a domestic travel within France with my partner, however he’s not a pass holder. 

The only way I’m aware of is for me to book on travel.b-europe.com for myself as a pass holder, and for him separately on other sites. Which would also mean we would not be seated together as we’d like. It’s quite important for us since we rarely see each other. 

My question is : is there any other way for us to book together even though he’s not a pass holder? I’m a new user so I don’t know much.

Big thanks in advance for all the replies!