Funny, I posted the same basic question a while back. Only it was about "Outside Shooting"
(156925.1).
Yes, in theory there's a *slight* advantage to training wingmen. So when not overly inconvenient, that's what I train. Though to be honest, my results have been just about identical compared to when I've trained JS for guards only. (both in terms of training speed & skills popped). Of course my sample size is small so keep that in mind. But again for me there's been no substantial difference to this point.
Btw, have you gotten many pops in IS while training wingmen? Just wondering because I *never* have... multiple pops in all the other secondary skills, but never in IS even one time..!?!