Radiator fan query

[cauzu]

What would happen if you had one fan push and another pull on the same side of a 240mm rad?

 

[R_1]

the hot air from one fan could be drawn into the other fan, the heat moving in a figure O (circle) increasing heat not dissipating it. if the radiator is on the top or the front of the case it could force hot air back into the case, the last place you want it.
Having both fans go the same direction allows the source cooling air to be shared by both and the exhaust to be away from the case.