I would go with the single column dual tap. Remember you have to cool the towers anyway so why don't ya take the easy way out and only have to cool one
Running two kegs off the same regulator is fine if both beers need the same CO2 pressure. I went with a secondary regulator so I can adjust each keg separately. If you use only one regulator and have it spliced into two kegs simply untapping the other coupler will stop the flow of CO2 into that side.
5 LBS cylinder will be fine for 10 full size kegs