Chad - Yes you can do exactly what your asking given the 4 / 2 ohm stable rating of the amplifier in question. But I would riddle this back to you so you can ask you self a few questions...
What is the intent of the tower setup? If its for the rider, you're not going to have very much sound back to the surfer or the wakeboarder. If its to add ambient sound you could run with the plan you've laid out.
The reason guys start multi amplifier system upgrades is because they break out the zones by function SUB / CABIN / TOWER. In doing so most of the time they require dedicated amplifiers for each zone. Yes there are some exceptions to this, as it depends upon the power levels the stereo is designed for.
Do you want independant control of the tower zone? In the setup your talking about you'd have to fade between front/rear unless you have a EQ in the system. If you had higher power tower speakers, I'd definitely recommend a remote fader control form a safety point of view. On the plan your asking, I don't think its really needed.
Second Battery: I think you're on the mark here... If you keep it simple with 1 amp, then yes, your safe with 1 battery. But If you find yourself wanting to sit, or wanting performance that dictates a second battery--- DO IT. its less than 100 bucks for peace of mind. If you do add a second battery make sure you've properly fused the amplifier upgrades for safety.
As a fall back position on your setup, if your amplifier isn't rated at 2 ohms, you might look at powering the cabins off the deck and using the existing amplifier to run the tower. Theres always a few ways to skin the cat on this stuff.
In the end, it comes down to performance level dictates all else. By discussing these subjects at the very start, it will hopefully save you time, money and troubles in the future.
Hope that helps a bit.
Last edited by brianinpdx; 08-21-2011 at 5:36 PM.