(Let me know if this makes sense or if im off my rocker). In a sub/sat speaker set it seems to me that having 2 subwoofers, one large one and one smaller(like a 12in and an 8in) makes sense. One could be more dedicated to the lower sounds(the large one) and one to the higher sounds just below what the satellites can reproduce. It just seems that it would produce cleaner sound. Also, why would a smaller sub sound better with small satellites than a large one, wouldnt the size account for some of the lost umph in the small speakers? Let me know if im just crazy and if im not, post your opinion!
Cody there is problem of connection. Most receivers have one sub out and even if you had two out puts chances are that there will only be one cross over point(where the tweeter stops and the sub takes over). If you use book shelf speakers with 6 to 8 inch drivers with a sub, your blend of highs mid range a nd lows will be all covered.
I've got two subs set up exactly as you describe and they sound great. You can Y the single sub out and run Line-level (RCA sub cable) to each sub, but make sure you Y it again to go into the sub. Most subs wont allow you to use the crossover setting on the sub unless you have two inputs going into the sub. Make sure to set the crossover on your reciever as well. It doesn't make any sense to have a sub at 200hz and one at 100hz if the crossover on your amp is at 100hz.