I remember the guys at SVS recommending, at one point, to combine the 1646 and the 2039 back before they had all the other models of SVS subs. One sub is tuned to 16Hz and the other is tuned to 20Hz. One has more output than the other sub, which has a lower extension.
The smaller the room, the more likely using 2 different tuning points might provide some benefit. We usually suggest sticking with like models in most scenarios.