I do not have the Scythe Mugen II, but I using two fans (push/pull) on my Thermalright Ultra120 eXtreme for my Core i7 build. At idle it keeps the CPU about the same within 1 to 3 degrees'celius lower then a single fan. When under load it keeps CPU about 10 to 15 degrees' celius lower then single fan.
Hope that helps you out.
Message edited by Yoosty on 08-03-2009 at 09:24:22 AM