I run a two faucet tower with beer and soda. I aim for 39-40 degrees and I run beer at 10-12 psi and soda water at 35. We mostly drink the soda water straight or mix it with crystal light lemonade powder . . which makes for a surprisingly good sparkling lemonade (seriously - try it!). 35 psi is perfect for that - where you're just adding powder instead of syrup. My dad just gave me like 25 bottles of torani syrup and I've been making italian sodas lately. You have to add a lot of syrup to make a soda . . and at 35 psi they come out a bit flat. I'll probably move it up to 40 psi.
I don't know how many volumes of CO2 that works out to - the beer tables I've seen don't go up that far.
Yes, you have to balance the system just like beer . . but foam isn't an issue with soda water. When I first hooked it up with 5 ft of 1/4 in line, it would fill a 22 oz glass in about 2 seconds in a quite violent fashion - unless I just barely cracked open the faucet. I now use 20 ft of 1/4 beer line on the soda side and that's perfect.