Yeah...for best results use two...what they do, is vent excess air from the ram, in either direction of its travel...so, rather than compress the gas, when the ram is opening or closing, it vents it, which makes teh ram operate much, much faster.
If its a 44/Nexus ram, with two qev's, increase your CDEL setting to 006 (if its and E1, its allready 006 on E2's)...this allows your hammer etc, more tiem to finnish what its doing, before teh ram starts to open teh bolt again..otherwise you will get blowback.
Ideally, you should set all your settings, to your gun.