Sharing a shower drain and vent

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

A 1½" tub trap arm could wye into a 2" line from a shower. Most likely the vent would be OK, would have to see a pic. Each item absolutely must have its own trap.

drain

How you make the connection will determine whether it is a good one or not. The same general description, such as you gave, could apply to both good installations and bad ones. And asking if both items need traps does not engender confidence that you know the correct way to connect the two together without creating a long term problem.