Back to back second floor bathroom DWV questions

[Spamtastic]

The back of the toilets share a common wall and connect to the vertical waste stack at the same level via a cast iron double sanitary tee and are currently the highest fixtures. The stack is reduced to 2 inch above this fitting and continues through the attic and roof. The bathtubs are situated lengthwise next to the toilets along the same partition wall. The p traps are each within 3 feet of the stack. I would like to replace the cast iron so that I am able to run the tub drains into the stack just above the toilets. Basically, A double fixture or double wye and 45 combo for the toilets and a sanitary tee directly above which accepts waste from both tubs via a single 2 inch pipe.

Can the bathtub drains be joined before connecting to the stack without separate venting? Or do I need to connect each tub separately to avoid additional venting?

Thanks in advance.

 

[Terry]

A double fixture fitting on the vertical to pick up both tubs situated above a Double wye for the toilets would work. Trap arms for a tub can be 42" long.