do the kicks sound the same when you use your dominant foot? do the kicks sound different when you use the other foot? same tension on the pedals? same type of drum head and padding if any? is one kick facing a wall and the other not? have you tried swapping the mics? are the same type of...
you can always look into the RM-7 from Stellar (stereo ribbon).
Stellar RM-7 Stereo Ribbon Microphone | eBay
or this one. http://cgi.ebay.com/Superlux-Location-Mic-E524D-X-Y-Stereo-Condenser-/180477534071?pt=LH_DefaultDomain_0&hash=item2a054c9f77