Depends..

Are you going to draw 400 watts from both of them continuously? If yes then you would be drawing about umm 67Amps @ 12 volts.. If you're not running a heavy stereo system or tons of lights you'll be fine.. Just don't keep it on too long with the engine off.

If anyone sees a mistake please correct me before he fries his alternator and blames me .

my 2 cents