I have a UHF and a CelFi Go aerial mounted on the bullbar of my 4wd. The other day a bloke was telling me that I would be getting interference between the two because he thought they were too close (on the same side, approx 15cm apart).
I dunno the antennas, but doesn't matter, Just put them as far away from each other, any metal and as high as possible. Bullbars are a stupid place to put them if you want them to work as far as possible ( and isn't that why you use an antenna). Your Cel Fi go is a complete waste of money if you don't put the antenna on the roof IMHO.
Actually the general wisdom is that a quarter wavelength of the lowest frequency - in this case about 16cm - is 'safe' and will avoid the worst effects such as overloading receivers and completely obliterating radiation patterns. Of course, Boobook is right, height is might, and more distance is better, so if you want the best possible results diagonally opposite corners of the roof rack is pretty hard to beat.