Two vent lines will work as far as vents go, however it's not practical if you use a taxi tank. You'd be replacing fuel in only half your tanks unless you use two taxi tanks.

One reason tanks balloon when filling is because the vent is too restrictive. Usually the most restriction is at the tee. A better idea is to use an oversized tee and an extra large line after the tee. The single line after the tee needs to be sized to flow twice as much as the individual tank lines.

I normally use 5/32" tygon to plumb my tanks. Get a 3/16" tee at your local auto parts store and force the lines onto it. After the tee use 3/16" tygon. Also if you have a fitting where it exits the fuse that also needs to be sized for 3/16" line.

Joe