you could use two, but you would have to get a y pipe and mount one e-ram on each end of y.
If you just put two in line you will not gain much more than just having one. The second would only add half the hp of original.
If you ad Y you will get twice the air folw and twice the pressure (pressure+more air=more hp).
Have fun, but it seams high priced to me for only 15 hp, my drop in K&N filter added 8+ hp
