I have two 12 inch Pioneer subs. They are 400 watt RMS each. I want a class D MONO amp.?

Be warned that simply "turning down the volume" will not save your woofers from being damaged by a very powerful amplifier. An amplifier plays as strong as it can all the time, regardless of any settings. The wattage that comes out is determined by the voltage going in and how the gain is set to match. So just by turning down the volume, you can still destroy a woofer easily.

The above is correct. What he means by the 2 ohm from two 4 ohm subs is when wiring parallel. http://i81.photobucket.com/albums/j230/sparky3489/PARALLEL.jpg the result load will be 2 ohm as long as they are single voice coil. Z = 1 / (1/4 + 1/4) When connecting subs together, the watts of both subs is added.

you need a 1000 watt amp for that

You can never buy a amp that is too powerful! You just won't turn the volume as high. Esentially the higher power amps will create cleaner power down low where as if you buy a lower powered amp you need to turn up the volume/power and this also creates dirty sound. You need to buy a amp that'll produce at least 800 rms at the ohm load you'll be using (4 or 2 ohm). But, I'd suggest buying something even larger.