I've always been a bit fuzzy as to what things leaders can do while they're being transported in a truck or halftrack. From what I see (or don't see) in the rules it appears their abilities don't change whatsoever. So if I have a leader with a combat modifier being transported by a SPW 251 that is adjacent to another SPW251, can he use his modifier to add together the DF numbers of the two SPW's to perform a combined attack?
You are correct as far as what you have said. The nuance to keep in mind is that while the leader can use his combat modifier to combine the halftrack's fire, he cannot add his modifier to the direct fire value of the halftrack he is with.
"The nuance to keep in mind is that while the leader can use his combat modifier to combine the halftrack's fire, he cannot add his modifier to the direct fire value of the halftrack he is with."
What rule calls out he prohibition of not allowing the leader's modifier to the direct fire value to the halftrack he is with?