I think alot of this answer depends on what motor your gonna be using. I have a H9 P-51 with a pumped O.S. 1.20 4 stroke that mounted totally inverted. My engine has a vent nipple on the back of the crankcase that I have running back into the tank,so I'm using 3 lines. I also use a "T" that connect to my tank/vent nipple/and overflow filling line(this engine doesnt have a pressure line in the muffler). I have one line going into the tank for the actually fill line. And of course the other is the carb line coming out of the tank.

But most likely your gonna NEED a 3rd line in your tank. "T's" arent bad if you put them in the right spot,but if you were planning on putting the "T" between your tank and carb.......that would likely not be good.