I have a similar situation, and am wondering the same thing.
The amp I'm planning on using is the MTX Thunder 421D.
http://www.mtx.com/caraudio/products/amplifiers/thunder421D.cfm
If you click on the picture on the above page, you'll get a larger image in which you can clearly see the three posts for power/ground/remote, and to the right of that, two pairs of speaker posts. (Right side of the image.)
I've been told that these four posts are paralleled for ease of wiring when wiring two subs/voicecoils for a 2-ohm load.
My problem is a matter of appearance, though. I'm going to run a single DVC sub off this amp. Normally, you would want to run four wires for this. I want to run a single pair, however, and wire a "jumper" across the speaker terminals at the speaker to get the 2-ohm load.
However, for appearance, I want to use all four posts on the amp. (I just the the wiring will look better if all connections have a wire in them, even if the wire is not necessary.)
So, what I'm thinking now is to put two pairs on the amp posts, then solder the positives together and the negatives together several inches down the wire, where they'll be out of sight. Then run the resulting single pair to the sub, where it will be wired directly to one set of posts on the speaker, then "jumpered" in parallel to the other set to get the 4-ohm load.
I don't see any problems with doing this for cosmetic purposes if, like you said, it's really a mono amp with two sets of posts. But it's been a long time since my days of doing complicated electrical diagrams, so I can't be sure I'm not missing something, here.
Maybe it'd be simpler for me to just run the sub off a single set of posts on the amp, and put a pair of disconnected dummy wires on the other two posts. //content.invisioncic.com/y282845/emoticons/wacky.gif.cf4a48f9e6dfe9504ffd2c946fd2f340.gif