No stud holes option: lift up floor boards, penetrate the bottom plate in two separate stud bays for the two separate drains, join them horizontal in the floor joists. Similarly for the vents/top plates/ceiling joists. Your horizontal drain looks pretty high in the joist bay, and holes in the joists have to be at least 2" clear from the top of joist, so this may not be possible.
Maximum copper (requires soldering) option: PVC 2x2x1-1/2x1-1/2 double fixture tee, short 1-1/2" pipe segment, 1-1/2" PVC x 1-1/4" Cu DWV Fernco, bore through studs with 1-1/2" hole, DWV 1-1/4" Cu elbow, 1-1/4" copper trap adapter.
Minimum copper (no soldering): same as above through the studs, then another 1-1/2" PVC x 1-1/4" Cu DWV Fernco, with a 1-1/2" x 1-1/4" PVC bushing glued on 1-1/4" PVC pipe, a 1-1/4" PVC DWV elbow, 1-1/4" PVC trap adapter
For the minimum cooper (no soldering) option, there will be waste line size changes from 1-1/4'' PVC to 1-1/2'' PVC, and to 1-1/4'' Cu, finally to 1-1/2'' PVC on the double fixture tee. Will there be any problem of size changes of getting bigger and then smaller?