If you are real comfortable with measuring response and setting up EQ filters, I think you could get pretty good results. My first try would be to let the SubMersive dominate over 30-40 hz where it should have plenty of headroom and better SQ. Use the SVS to smooth the room response in that range if necessary and to add headroom to the region around 20 hz, say 15-30 give or take. Finally, the SubMersive will handle the very deep stuff, although you could experiment with the ultra in the deep tune or sealed modes.
If you try to have both subs contribute equally, one will hit its limits before the other, and you will leave headroom on the table. OTOH, with what I described above, you leave headroom on the table as well. In other words, you have to make compromises with a dissimilar sub setup.
Thank you mfine. I'm not really comfortable with filters and planned to let something like the SVS AS-EQ1 do all the EQ stuff for me. Do you think this kind of automatic calibration process could give good results with dissimilar sub setup for a calibration impaired like me?!