The question of one 2 faucet tower or two singles is entirely your call. Go with what you like.
I would think that a converted freezer should have the cooling capacity to cool 2 towers.
Be aware that you will need to build a fan cooler for both towers and will neeed either 2 drip trays or a larger tray to fit the tower configuration.
I personally would go with the single 2 faucet tower, for no other reason than I like the look.
As far as the regulator goes, the one pictured in your link has 2 shut off valves and should work just fine. If you really want to better contol the pressure on your 2 beers, I recommend adding a two product secondary regulator set inside your cooler.
http://www.micromatic.com/draft-keg-...-pid-8022.html
I do not recommend trusting that simply leaving the handle up to shut off your gas, I guarentee that you will bump the handle and loose your gas very quickly. I know, I did it.
Enjoy your kegerator, which ever way you build it, toss one back for us!
Dennis