you cannot. the ohm rating on a sub is what determines which possible wiring loads are available to the amp. wiring in parallel will cut the ohm load in half, while wiring in series will double it. so...wiring one dual 4 ohm sub in parallel will make it a 2 ohm load on the amp. wiring a second dual 4 ohm sub in parallel will give the amp a 1 ohm load. being that they are 400 watts RMS, you'd want to get an 800 watt RMS amp @ 1 ohm, so when both are wired to it, they are each gettin their rated 400 watts.
just another example, if you have a dual 2 ohm sub, wiring in parallel will give it a 1 ohm load on the amp, and wiring in series will give it a 4 ohm load. wiring two dual 2 ohm subs in series will be 8 ohms. see the pattern? somewhat basic, but can also get complicated with more subs in the mix