I want to add a tub to my bathroom, I only have a shower now. I want to keep the shower and add a tun next to it can I use the same drain for both?

i want to add a tub to my bathroom, i only have a shower now. i want to keep the shower and add a tun next to it can i use the same drain for both? It won't be code but the tub will use the same wet vent as the shower. The only problem with that is when you drain the tub you stand a very good chance of it backing up into the shower since the tub will have head pressure behind the discharge. Your choice but don't say that you weren't warned. Good luck, Tom