2 identical cabs are the best option, . An 8 ohm 410 paired with an 8 ohm 210 means that each spkr in the 210 get double the power as the ones in the larger cab. Makes the 210 kinda pointless. A 4 ohm 410 paired with an 8 ohm 210 balances the power between all spkrs, but the amp needs to be 2 ohm stable. Really, another matching 410 is what would work best in this case.