If I'm not wrong.. correct me please, the front and back torp. man are independent from each other. They're just like the gunners on a BB/CV, but in this case they fire torpedoes.
Im not sure how much vets/experts to put..because you don't wanna put too much...
space on a sub is very very valuable. 

Simply put, you want your best torpedoman in the front slot. 

Max everything.  A microsecond of reload can be the difference between destroying your enemy or being sunk.

Put your best vetted torpedoman in the front. As Ultra_dog mentioned, every millisecond counts when going up against other ships, especially sub vs sub. The rear torpedo tube is not as important since it is not used as often as the front. You didn't mention what sub class you're in but in general, as your crew grows you'll quickly start reaching the weight limit for your sub. What I've done with my rear torp sailor is to "ghost" him a bit by decreasing the number of rookie recruits that way I can allocate recruits in other better suited areas such as my planesman, front torper, engineer, etc.