You couldnt just run it off the current one,
You would need a distrubution block to split the power from your current power cable which you could then use to power both amps,
http://cgi.ebay.co.uk/Vibe-FD4-4-way...als_Cabling_ET
Something like this should do, i would go with a fused one for extra protection.
you will need to run a big powercable at least 4AWG from the battery to make sure your amps get enough power.
You would still need 2 earths and 2 Phono cables.
This wiring diagram should help
http://www.saxperience.com/forum/showthread.php?t=67174