It depends on where you mount it, but if you have a Driffter/ODL mount, a 4' looks/works good. IMO. You'll most likely be snagging a 5' on branches like crazy no matter where you mount it. On my CBI bumper, I was snagging a 4' a lot so I changed to a 3'.
I have the driffter mount and 4 foot firestick. You can drive down the street and see it if you want.
Here is my 4' firestick on the Drifter mount. I am not sure why you want to run 2 antennas. Our trucks are not wide enough to have proper seperation and will end up with a very directional signal. You will be fine with just the one. If you want a second antenna add a Ham radio.
4' fire stick mounted on front bumper:
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I like having it on the front bumper. Without the roof rack it is the same height as the roof, so I use it a lot as a guide for getting into parking garages. Its a good measuring stick.