I am putting in a whirlpool tub that will be in line with the shower drain, can these two share a drain and vent, do both need traps, I am making the shower bigger and have to move the drain out about 6 " to be in center of shower so can I 90 off the new drain to the old one for both units, also what pipe size from new tub drain to old shower drain ( 3' length ) any thoughts or advise is welcomed. Thanks, Tim