Will these 2 12" subwoofers with an 1800w Amp?

Yes. If they are 400RMS subs and 4ohm single voice coil, then you can wire them together in parallel to create a 2ohm load. The amp is 900x1 RMS at 2ohms so you would be putting about 450watts RMS to each sub. Slightly overpowered but no big deal, just don't turn up the gain super high.