Okay, I'm going to show off my ignorance again...because this doesn't ring true to my ears. (You and Gene can come around later a smack me around a little.

) Certainly identical subwoofers are ideal in multiple setups. Agreed. In significantly different subs, there is undoubtedly a notable and easily recognized difference in output qualities and capabilities. But that is not truely the case in the OP's situation. These are relatively similar subs.
Gene's article states concerns about mismatched subs phase and amplitude issues. Not much of a problem with these two subs. Phase and amplitude can be individually set and matched (as with any identical multiple sub setup this can be an issue). And the same "mismatch" argument can hold true regarding resonance cancellations and standing waves in non-co-located identical subs.
Secondly, as a sub begins to distort at its lower levels of reach, what is the difference between these subs' F3 point? .... 5 Hz? I think that is likely an insignificant disparity, especially in an HT presentation. So the 1200 will limited to the 1000's quality performance level a
little bit but the net headroom gain seems worthy of the addition.
Again, the OP's question raises issues of ideal vs. acceptable audio setups. There is a shade of grey here that seems worth his trial. And the price is absolutely right...it's free he states. I say "Try it!"
EDIT: After posting I saw Annunaki had just posted. It's a fine idea and suggests the best use of both subs.