Code doesn't allow putting those two fixtures into one trap.
I saw a similar remodel where the handyman had done that. When the tub drained, black goo came up in the shower pan.
The walls will be out for this remodel. Figure out a way to install a vented trap for each fixture. You could get by with a double fixture fitting perhaps and use one vent, but it's not a big deal to come up with two vents and revent higher up either.
ABS or PVC pipe and fittings hardly cost anything.