Sell the L5, get a better box for the L7. As big as you can fit. It'll make a world of a difference, more then cramming another sub in there. You don't have enough power to run both, and they do not match. How ever the L7 and the one amp match well and getting or better yet building a bigger box is cheap and will yield much greater results.
Wire it to 1 ohm , not 4. Make sure you have decent wiring and a half decent battery too to run 800-1000w rms.