You'll only need one amp for the sub. Either a 2 channel bridged or a Mono channel amp. In your case, you are using a single 15" sound quality sub. Therefore you would want to use a 2 channel class AB amp, as opposed to a monoblock class D amp.
Better sound quality with the Class AB.
So, get the 2 channel amp, and bridge the amp.
Review the owners manual for that amp
http://www.diamondaudio.com/products...0.2-091902.pdf
You will notice that in order to bridge the two channels of the amp you would use the left channel output positive connection for the speaker positive, and the right channel negative output connection of the amplifier to the speaker's negative post.
You will have to make another jumper wire to wire the subs dual voice coils.
As seen in this pic.
Since you have a dual 2 ohm sub, your amp will see a final load of 4 ohms. Which the amp is rated to handle when bridged. Put the sub in a sealed box with about 1.35 cu ft airspace.