I can't figure out what you have going on here...
"...shower and overflow drain is a single pipe..." ????? Do you have a shower stall AND a tub and 1 pipe? Is the pipe horizontal? Is it vented? What size is it? Where exactly is it? Below the concrete? How far away from the shower and/or tub is it?
"...need to add a T-connection to run under the tub for the shower drain and continue up to connect to the overflow..." Huh?????
However, something is not right....
If your basement was roughed in for a full bath, you shouldn't be going through this unless you framed out the bathroom differently than the rough-in was done (in which case you almost always need to remove concrete and move drainage/vent/trap arm lines). There are restrictions on how far a trap can be located from its vent. Every fixture must have a trap and be vented. You cannot use a tee in the horizontal. If you have both a tub and a shower, you cannot hook both to this line if it is a trap arm (which it probably is).