As long as one's not masking the other and they can both "see" the sky I can't see it makes too much difference. They're both recieving aerial's. If one or both were transmiting aerial's I think things would be different.
GPS antennas 'look' vertically upwards at the sky right down to abt 10 degrees above the horizon below which sats get swamped in ionospherics. So imagine a huge cone pointing upwards and try not to interfere with it. In practise, you may not be able to but provided your obstructions are small you'll be fine as there are plenty of sats about so blocking one for a few secs isnt a problem.
GPS antennas are active, but don't radiate so will cause no interference if close by. Try to get them at the same level (so they stay below each others cone) and 4x the antenna diameter apart. Put them above or below the radar but not at the same level.