DWV design sanitary tee

[FLhome0wner]

I have a 3 inch water closet that drains about for about 3 ft to the the lower part of the stack with a vent upstream of it. I am adding a new drain that will be coming from the opposite side of the stack (180 dregees from the WC), it's vented elsewhere. I am trying to keep them both within 8 inch joists. Can I use something like a double sanitary tee with a cap on top? Or a double 90 degree elbow? Or will I need two sanitary tees, one on top of the other? I was thinking something like this.

I've attached a photo of the current plumbing but the joists have been redesigned by an engineer and the plumbing will be moved up between them.