I have individual TB on my car, Im plumbing in the fuel rails and am not sure the best way to do it.

I have a ''Y' fitting on my feed line so each rail gets a direct feed from the pump.

do I have to return both rails to the regulator or can I plug one of the rails?

 

You should return both rails to the regulator. Then have a single line back to the tank.

Most regulators have duel inputs. If yours does not, you should Y them back together before the regulator.

Although it should work the other way, because it is a pressure system. It is not optimal.

You could run one rail into the other, in a series layout as well. Then you only have one return line as well.