Okay, Basically. The sunk-in shower is obviously lower than the bath tub. And I wish to connect the new drain from the tub to the existing 2" drain from the shower. My question is: How would I go about venting (if needed) and connecting the new tub that is 4' away from the existing drain, Without, having any suction or backflow to the P-trap of the shower when we drain the bathtub?