You mean so that the 2 fans are actually touching each other? I wouldn'trecommend that. AFAIK the turbulance between the two blades would be a problem. On on each side or a 1/2inch buffer between them (say a dead 120mm fan with the blades removed) would help.
Pressure increases your ability to approach the rated flow rate of a fan. Higher pressure means one fan will work closer to its rated max. If you can fit 2 fans side by side it will always do better then 2 stacked. However its very rare to fit 2 120mm fans side by side.
|