Adding bathroom back to back with existing - share 3 inch branch?

I drew up a crude picture of what i am looking to do. What i need to know is if each toilet needs its own vent? how many 45 deg bends and wye in horizontal run? I would like to arrange the existing 3" branch for both toilets. If not I will have to do much demolition in the finished basement. I am adding a new bathroom back to back to the existing bath. The toilet drain is tucked horizontall between the joists. I am looking to do this:
after vertical rise - leave exiting 90 elbow - leave existing 3" T vent up...then add 45 street elbow to 3" wye - straight wye leg to 45 street elbow up to toilet. other end of 3" wYe to another 3" wye to other - wye leg to toilet then straight leg to tub with cleanout. Very hard to explain. Any help would be appreciated. cant get photo to show so here is the link:http://www.flickr.com/photos/31031106@N08/2899135119

bathroom

It is hard to visualize the fittings you plan to use, but if the drain is between the joists you WILL NOT have room to install the pipe right below the toilet and go up into it.

Thanks!

Thanks for the info....thats what i needed to know....it's more than 10" so i am running a complete new horizontal branch for the new bath with new vents for each fixture...

drainage

Post a picture when you get done, so we can see if you have to take it all out and start over. Installing a "vent" will not always mean that the traps are properly vented. HOW you do it is more important that WHAT you do.