1.)
Welcome to Open Roads Radio!
2.) I have no clue.
But we have some pretty high powered nerds here and I bet one of them will have an answer for you. What exactly do you mean by "dual" ? You want to run two of them in phase? (or out of phase, depending) Any two (or more!) vertical antennas can be placed in a phased array for directivity, if that's what you want to do.
Hang tight, some one will be along with some kind of answer.