Bit of a random one for those with slightly older twins. My 4 year old girls are interested in learning to play the violin. Just wondering, is it better for them to have separate lessons or a joint lesson? Normally I'm all for separating (they're in different preschool classes), but in this case I think they'd be dying of curiosity to see each other's lessons and will probably want to sit in. I also think being together might help them to keep focus and might spur them on to practice. But then, separating is good in school so does that also apply to music lessons?
They are identical and not unequal in cleverness, so I don't anticipate massive gaps in aptitude that might cause problems in a joint lesson, but perhaps that's a naiive of me! We did toss out the idea of different instruments, but they seem to be set on playing the same thing. 
Does anyone have experience with this? What did you do and/or what did the teacher advise?