Running 2 different subs, and what size?

There is no comparision between your sub and a DD-18. I own one and they are just increadible. The HGS is a fine unit but the Digital Drive series are in a leauge of their own. Get it and see!
I think you may be able to daisy chain the HGS to the DD and have the EQ balance it all out but am not sure if you can for a non-DD unit. However, that may not be necessary at all after you hear the DD.

You will more than likely need a good parametric E.Q. and some good setup info and sub position in the room will be much more criticle in a stereo sub situation.. I tried it before with 12's and it was better with them stacked on one side together opposed to across the room from each other, but in reality an interesting type setup would be to get a real Fast 10" sub for tone, and maybe a Brute 15" and stack for a complete solid coverage and 2 drivers from the same manufacture would cover the whole range where you could get the room boom out of the 15 with deepth like feel, and the 10 would keep things faster and musical, of course both would be crossed over a bit different, probably like 40 hz for the 15 and 80 or 100 maybe for the 10.. If stacked you would get just about the same bass as across the room from each other and also be able to more than likely forget about having to sink everything in with a parametric.. If I was to go back to those days this would be the way I would approch it.

With two dimensions being the same you definitely have a room mode issue. We all have them but when two dimensions are the same the problem doubles. This will make cancellations and peaks twice as bad as usual and the remedy is bass traps. Another sub in that room could make things worse than they already are.

You should either go to a place like realtraps.com and buy some but while you're there read about acoustics and watch Ethan's videos. You can make bass traps also but they're not as good as the traps realtraps sells.

This is the best advice you can get I can assure you. Your listening position may be in a null at least I get that impression from the way your post reads. A bass trap will absorb soundwaves that are reflecting off the walls in your room so they can't cancel each other out or create even higher peaks. This will also increase the clarity of the midrange.

A bass trap will make the amount of bass in your room louder, more articulate and accurate. It may seem contradictory but that's how it works. If you go to a corner of your room and listen to the bass and then sit down in your listening position you'll notice a large difference in apparent bass.

I just had an email a few minutes ago, here's a copy and paste of part of it:"Room treatments on the other hand were huge. I have 14 GIK panels in the room now and the improvements were easy to hear. Mostly in the bass. Where I really had none at all, it is pretty damn good now."

This will make more difference than any amp, preamp, dac and especially wire could possibly do by a wide margin too.

Hope this helps. Cheers