Yes, it would be the same difficulty as "Part."
Individual and Group are separated by 2 magnitudes; logic suggests that, somewhere between 1 person and 10, there is another Target.
Of course multiple casting can cover 2 people. It can also cover 3, 4 or 5 people depending on your level of Mastery. It can even be used with Group spells, allowing you to cast the same spell on 2, 3 or 4 groups! A "Target: Pair" spell cast using Multiple Casting would function exactly like a Target: Group" spell with Multiple Casting, except it would only affect 2 targets instead of as many as 10.
Presumably, yes, the pair would need to be distinct somehow, much like Group.
Breandan would be exceedingly interested in finding such a mouldy old tome. In fact, he's considering spending a fan point.
Let's not derail Dunia's thread any more than we already have!
A +1 smaller group is an awesome idea. Pair is fine, though Trio scales better. (+2 for 3*3=9 is close to 10)
FFM becomes slightly more valuable. (I like the virtue, but it seems unpopular, and never makes it into any of my builds either, probably because FM and LLSM better satisfy power and flexibility, respectively. Heck, FM itself gives power and flexibility.)
I'd go with either the original research rules, making copies of the same spell to do the research or having a requirement of it being your 5th/6th mastery on a particular spell, to display the idea that you understand it so well you can come up with new ways to modify it.
Reaching that level of mastery is quite an investment, which explains why new masteries aren't common.