I guess one is a vent and how do you vent , the big one is the toilet, and I guess one is for a tub.

i guess one is a vent and how do you vent , the big one is the toilet, and i guess one is for a tub.
There should be thjree pipes and not just two.
A 3 or 4" pipe about 12" out from the wall is the toilet stub-up.
A square 12 X12" hole in the slab with a raiser is for a bath tub and there should be another pipe next to the wall to pick up a lavatory. The pipe hanging down sounds like a vent. This pipe should go out the roof on one end andconnect to the lavatory with the other. Can you take a picture and send us ? Regards, Tom