I've taken a cheap RCA splitter (two female and one male), plugged it into the Sub out and then ran cables to each of the subs I was using. Not only did it work great, it was less filling, too.
Since the subs are active or powered subs, splitting the sub output is completely invisible to the receiver.
Now, if you fancy yourself a golden ear, you may notice a tiny amount of sonic degradation, but this is extremely unlikely given that you're talking about frequencies in most cases below 80 Hz (or, in my case, since I cross over at 50 Hz, below 50 Hz)..
Should work just fine.