Marine pumps are the best....this is a good little pump...
http://www.harborfreight.com/12-volt-marine-utility-pump-9576.html..
I got one of these laying around that I have used for different things and it has never let me down...
If it were me...I would do 1 of 2 things....
I would either plumb in a separate water box...with a float switch..to run the transfer pump.....
or I would tie the float switch into the one I have now and use the float switch to turn the transfer pump on and off....
demand pumps always seem to have problems...transfer pumps..hold there own and last longer...
very simple to do either way...and way more reliable....