according to the fan thread if you want to stack fans you would need to space them about an inch apart. One way of doing this would be to cut the center out of a 3rd fan and use that as a mounting bracket. https://www.icmag.com/ic/showthread.php?t=131842
Nevermind I just got a cooltron 110watt 120 mm that blows 7plus. Also, i really liked Dr Zoidbergs carbon filter fan sandwich in his outstanding Micro Grow PC journel that we should all praise. Might rig mine up something like that.