Provided they are not sat next to each other, with antennas in exactly the same plane/alignment and you are not pushing to the limit of range you should be ok for practical day to day use. GWs are somewhat able to ignore each other by design and implementation but close proximity operation does raise the local noise floor and reduces sensitivity a little in real world use and if very close (say <2-3m without intervening absorber) can risk saturating each other’s front ends when tx’ing. I often have 6-12 GW’s mix of indoor & outdoor running in a small volume roughly 15 x 35 x 20m, though with some intervening walls, glazing and floors, depending on what I am testing/commissioning and burning in at any given time. If you can ensure several meters apart preferably with an absorbers such as a wall or two between them that helps as does offsetting vertically. Directly above/below is good as the ant propagation is usually poorer in that direction. If you are running with feeder cables of any decent length and quality rather than directly attached that helps to have gw main units that in close proximity (within a few metres) effectively better isolated by ensuring the antennas can then be further spaced apart and offset vertically. I have 3 GW’s placed in the roof space at the moment on long term tests, one with feed to an external ant and the other two mounted under roof but, one direct attached ant and the other into a few metres of feeder cable, and with several m separation and also offset wrt the external ant. The external unit is slightly higher and being external has the greatest range/sensitivity, but the two internally mounted have similar performance, and allowing for the through roof losses are not too far behind the external unit. More importantly they have broadly the same real world coverage tested out to 4-9+km depending on direction as main limit is the local topology and line off site issues caused by being on a hill on the edge of the Thames Valley… I can’t see 20 or 30km from here so am never going to push these units to extremes 
(unless servicing a high altitude balloon or a plane out of Heathrow! )
Best way to find out is try it and test coverage 