Mr. Cable W,
I think I might of followed what you said but not totally sure. Cut the exsisting connection off and start fresh. You should use a double wye in this instance. Confirm the size of the pipe penetrating through the slap(most likely 2" per code underground.) Purchase a 2x1/1/2 double wye, 2- 1/1/2 street 45's and a 2x1/1/2 flush bushing if the vent required is 1/1/2. You may need to attach to existing with a fernco or clamp all. Set the center of the drains after double wye is assembled at 17 1/2". By the time you get to your location on both sides you should be at 18" to center of stub out. Rough waters at 21" and you are golden. SWEEEEEEEEET! I forgot get to regular sweep 90's for your stub outs.Good Luck Chuck!