I have put a new tub and separate shower in. Well, when we drain the tub the water goes out the main drain but it also comes up into the separate shower why

My guess is that there is a partial stoppage in the line between where you tied the two together and the main line. Water seeks it's own level so when the water is higher in the tub and has some restriction in the line it is going to seek it's own level by rising into the shower. It may never get as high as it is in the tub because some of the water is getting by the stoppage.

You can purchase a 15 or 25 foot cable in a spool and take the ploate off the tub at the over flow and sun the cable down the drain through that hole. This should solve your problem.

If this information helps please rate my answer.

What is the size of the drain for the tub and the shower ? How are these two drains connected ? Two separate vents ?

What is the size of the drain for the tub and the shower ? How are these two drains connected ? Two separate vents ?

2" drain for both. Pvc pipe from new tub to new shower to out the main drain, only 1 vent not two. I took my first tub bath this evening and when I opened the drain for the tub it goes over to the shower and comes up also going down the main drain. How do I stop the water from rising in the separate shower I installed.