connecting shower and toilet drain

I am busting up some concrete to move a toilet drain, and was wandering if, while I have the concrete out, I can tie into the toilet drain line with a shower drain. This would make life a bit better, as it would keep me from having to bust up alot of concrete and changing out the main stack. thanks

Yes you can, provided you vent the toilet first and connect your shower downstream of that vent.

So you will see, toilet flange, toilet vent and last shower drain

the toilet vents thru the main stack, so I am guessing that the new shower drain will also have to go into the main stack.

vent

Bad guess. Unless you have an unusual setup, the toilet will NOT, or at least SHOULD NOT, be vented through the main stack, because it will be a WASTE stack, not a VENT stack. And you have to vent the shower individually from the toilet. Just because something is installed "near" a pipe does not mean that it is vented, nor vented properly, nor vented effectively.