If you like your Avatar, get another identical model. 2 identical cabs (from any manufacturer) are going to sound better than any mixed cab setup.
If you don't care for the Avatar, then sell it & pick up 2 other identical boxes. The TCE RS line (210 or 212) would be good choices. But the best option in the price ballpark of two of these would be to go with a used 15/6 cab that employs the 15" Eminence 3015LF (low frequency) & one high quality dedicated 6" midrange driver (most commonly, although it's uncommonly great, the 18Sound 6ND410).
This 15/6 will outperform any commercial 2x210 tandem. It will demolish most of them.
ALWAYS open to sale/trade discussions on my gear...except the LDS 215, lol.