Shelley, I had to have a double crate made specially for my three wheeler. I can search out the name if you want to go down that road. The man lives in Stafford.
The only reasons I would not crate little bs with big bs is
(1) I don't want any fighting so you would have to play that one by ear
(2) If the little one potties in the crate accidentally the big b has to suffer too.
But I only have 1 so thats all I could think of
I use a car harness with My isZi because if I don't she wants to sit on my shoulders and it makes it very hard to drive. I have no idea why she wants on my shoulders even if I'm sitting at the desk on the computer she thinks she has to be on my shoulders. I guess because her ancestor's were carried that way?