Do I take from your note you intend using gutter mounts?
If this is the case might want to have a rethink. The gutter is only 10mm wide and if you have a UHF/VHF whip on it, the loading can be quite a bit.
Also might want to consider the separtion of the two antenna. If they are too close you can get QRM on one while the other is Xmitting.
A roof bar might be a sensible approach then you can remove them easier at a later date. I have mine on a 6mm steel plate bolted to the roof rack across the rear of vehicle but that might not suit all folks.
Good luck
T