Need Help - Don't Understand Ohms

I knew I should've paid more attention in math class!!  The more I research my problem, the more confused I seem to become.  I recently traded in my 10s and installed a pair of 12" Kicker Solo-Baric L5s, both 2 Ohm, Dual Voice Coils.  The amp that I have is an Alpine MRD-M500 (which was previously pushing 2 tens).   The owner's manual for the amp lists speaker impedance at 4 or 2 Ohms.    Anyway, since this amp is only 500 Watts, and each Solo-Baric will handle 600 Watts, I wanted to add a second MRD-M500, and have one amp pushing each woofer.  The guys at the local radio shop said I cannot do this because of the Ohms.  I'm not sure if they're wired in series or parallel (they were already in an enclosure when I bought them).  I would much rather spend $300 on a second M500, but they're telling me I would need to upgrade to the much more expensive M1000.  This Ohm stuff is really confusing me.  Can anyone shed some light on this to help me better understand it?  Thank you so much.

Thank You.......that's the easiest to understand explanation I've seen yet.  It is pushing both of them now (and doing a pretty good job of it)......I just have the Tim Taylor Syndrome......More Power!!!    I'm looking through some tech pages for the amp now to see if I can make any fine tuning adjustments.   Back in the day, we didn't have all this Hi-Tech stuff.......we just bought it and hooked it up.   Sure has gotten confusing over the years......thanks again for the help.   Oh, can anyone tell me how to get and sustain 14.4 Volts to the amp?