Hijacking the question to JCZ -
Corbin also makes a "slim trunk rest," shown here. I like the more streamlined look, and my gal actually likes not having the arm rests (feels less "boxed in"). When we ride 2-up I usually take my backrest off, because it does make her feel a little cramped. We both do fit, but it's less comfy with my backrest between. I'm 6' and lean, she's 5'8 and curvy.
The rider backrest just slides in an out, so easy enough to ditch (or you can bolt it in if you want).
Keep in mind the tourpak has two different mounting positions (onto the hardware), forward or back, which makes about an inch or so of difference. I mounted mine forward, because I ride 1-up much more often, but I am planing to re-mount for the sumer touring season so she'll have more room.
Sorry I don't have a better profile pic.