Two is already not that easy to calibrate, and the more you get, the harder it will be to get them to work together. If you want more bass, consider just getting one GOOD sub rather than two lesser ones. Two identical subs will give you exactly the same sound as one, only twice as loud (+6dB). They will not go lower, only louder.
Splitting the signal does lose a little voltage to both, but I couldn't tell you how many times you could split the signal before it became a problem, but as I said, lots of subs isn't a great idea anyway.