I just got the top case backrest for the 46lt box.

Do you guys think it's possible to combine it with the usual TMax backrest?

Doesn't look like it! And if you did, it looks as if it'd be pointless as if the passenger's behind was on the bum rest, their back would never touch the backrest!
I guess there's only one way to find out....

The interesting question is, if you can't have both, which is the most useful??? I must admit I've been considering removing the passenger bumstop from mine as it gets in the way of bags etc on the pillion seat.

It's mostly curiosity at that point since I haven't taken a ride with the wife yet to hear her opinion.

I will post an update as soon as we try it out.