Both subs don't need to be the same.
Personally, this has been my upgrade path for a 3500cuft room:
HSU VTF-3 MK5
Then Antimode 8033S-II (wasn't totally happy with how the Yamaha YPAO was doing)
Then additional 12" Aperion I had from another system.
Then additional 12" Blue Sky from an old studio system.
(Subwoofers setup using the Geddes method
and REW for placement and levels, then the Antimode)
The bass is as close to perfect across the entire listening area as I can imagine, but the graphs aren't perfect, so I'm still experimenting to see if I can get it even better.
Unless you have top-notch sub calibration from your receiver, I would prioritize an Antimode over a second sub. Get one sub and the Antimode, and then keep an eye on Craigslist, Ebay and the AVS Classifieds for robust subs in the $100-$250 range (I've seen older HSU, SVS and other quality subs pop up in that price range). Then you can add on as it's convenient.
Also, if you're prone to endless tweaking, the options in the HSU can end up driving you nuts, because no matter how good it sounds, you'll always wonder if a different configuration might sound just a little