Currently I am looking into the problem of using a common antenna for
two VHF transceivers. One operates 30 - 90 MHz and the other from 30 -
512 MHz. They would both be tuned into seperate frequencies and need
to share a common broadband, omni directional, vertical monopole.

I think I am getting myself a bit confused (or should I say more
confused), do I need to used a diplexer? Would a T piece not suffice
as both sets would be tuned to different frequencies and seperation and
harmonics would be considered? Would these factors not mean that the
input impedance to the set tuned into the different frequency would
remain high thus not effect matching?