Hi
I think that as long as you keep the 90 degree angle at the servo and the surface( at neutral) having them directly back to back wont matter. My preference if you are really tight for space is to have them staggered. One further forward than the other on opposites sides. One thing I would watch is to have the pushrods very stiff as the longer of the two will be prone to more flexing then the shorter.

That is my two pence worth.

Mike