I am renovating my bathroom and I want to install a new tub/shower combination beside the toilet. Can I tie the drain line of the tub into the drain line for the toilet? My other option is this, I had a corner shower on the opposite side of the bathroom, if I have to I think I can tie into it and come all the way across the bathroom to the new tub installation. This is about 16 feet away and I worry that I may not have enough drop by the time I get there. If I tie into the toilet line it is only about 5 feet away

Tying new tub/shower drain to the existing shower drain is better of two options. For 2" drain, you will need about 4" of slope to reach the old drain. You will also need vent for new tub/shower. That vent can be tied to toilet vent about 48" above the floor, or you can run it to the attic and connect it there. Hope that helps. Milo

First thanks for the quick reply! My next question for you is, why do I need to vent the existing 2" line again? It was already vented for the old shower. When I create this new vent line do I need to abandon the old one?

2" tub/shower drain has to have vent no further than 5' from the trap. Your vent is 16' away. You can keep the old vent the way it is. Hope that helps. Back to you. Milo

Do you have a bathroom lavatory? Is it vented? Check with your local Building Department to see if wet vents are allowed(see image). If they are you could tie to the lavatory drain and your tub/shower would be vented. Sound like a plan? Let me know, Tom