When
cutting the feedline for a dual band (2m/70cm), for maximum performance, it
needs to be cut to multiples of electrical half wave length. Does it need to
be cut for 2m? or 70cm? a combination of both? or split the difference?

As an old friend of mine (yup, I've got friends...) once said, "Make the coax
feedline just long enough to reach the rig."

Fits most (but not all) situations.

A
"...if one tolerates bad manners, they grow worse. I must find the oaf who did
this thing, explain to him his offense, give him a chance to apologise, then
kill him." (R.A. Heinlein, "The Cat Who Walks Through Walls")