First, to connect a second sub to the sub preout, just get a Y adapter (1M/2F). Plug it into the AVR, then run separate cables to each sub.
You will have some issues trying to blend a more capable sub with a lesser sub. As the lesser sub approaches it's tuning point, it will fall off quickly. This will leave the better sub all alone for the lowest frequencies. It will seam like the roll off at the low end is very steep. If you don't mind this, then you will be fine.
eD gives the same FR for both sub, seems strange, so you may be fine mixing the two.