If you like your DB115, then get another one. Identical pairs work best together. Any benefit (volume/displacement) from a 115/210 wouldn't be as great that from a 2 x 115. And a 115/210 (any non-identical cabs) would result in phase & dispersion issues. Would these issues be noticeable? Would they sound bad? Maybe not. But maybe. You'll know when you hook'em up. These issues are avoided altogether with identical boxes.
If you don't care for the DB115, sell it & pick up something you do like. Then, if you need/want more volume/displacement, double up on that cab.
ALWAYS open to sale/trade discussions on my gear...except the LDS 215, lol.