How should I do this?

Marty53 said:

combo wyes aren't the same as fixture cross fittings like the one you have installed. just break up a little more concrete on either end of the 4" pipe, use a small section of 4" pvc to connect two 4x2 wyes together and install them

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It's not a cross fitting that I have installed, it's a double-wye fitting, which appears to be consistent with chapter 7 of the code as I pasted above. Why is this not correct?

i think you're problem is that using the double wye in this case is necessitating many extra fittings and not giving your drainage enough slope into the drain because you have to lay the fitting flat. with two seperate 4x2 wyes you can angle both of the the 2" wye's up, and then run a 1/8 bend off it to make both of your 90 degree bends. it would flow better and use less fittings.

No, I do not "have to lay the fitting flat". It is graded just as well as the 4" cast iron pipe that it connects to. Thus, the two 2" branches that come off of it horizontally, yet are parallel with respect to the 4" section, are also sloped at the proper grade. Any piece of pipe connected to either of the 2" branches is therefore also sloped at the proper grade.

So what is the problem with using it?

If you slope one out let at 2% grade going up, the other side will be 2% grade going down.

Or you can set it flat, and see if your plumbing inspector lets you do it.

Any change of direction more than 135 degrees needs a cleanout.

Any vent below grade needs to be run with waste fittings.
No Santees on their back or on their side.

The vent for the shower needs to be pulled off within five feet for UPC
IPC may let you do six feet.

hj said:

I would be more concerned with what is happening outside the pictures as far as the drainage is concerned. Here, the double "Y" would not be a factor, BUT every inspector I have ever know, and every code I have ever read, does NOT permit solder joints under concrete, and since "brazing" the joints is a "destructive", and time consuming, practice, we NEVER make any joints in copper below the concrete.

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Let me ask this...if I switch to PEX and run it continuously under the slab, I will still need to attach the PEX to the copper which already exists under the slab, and will need to do so with a FITTING.

So how can I possibly plumb these two bathrooms cost effectively without having a fitting that would be under the concrete slab?