Although I've read a lot about double fixture fittings vs double SanTs, I havent come across anything that specifically says a double fixture fitting can be used on a horizontal drain pipe. Just to be absolutely sure, can I use a double fixture fitting on the horizontal to connect back to back toilets under 2015 UPC code? The toilets will be directly back to back, so their drains will be about 30" apart. The soil stack is about 8 feet from the toilets.

No, you can't use a double fixture fitting horizontally.

what would be the alternative? I want to have back to back toilets on a horizontal drain over a finished basement and have limited head room in the basement- the drain will have to run under the joists - which are perpendicular to the run of the drain pipe.

Use either a double combo or a double wye. Double fixture fitting is different than a double combo.

don't forget you need room to get a "Y" on each side for venting the toilets.

Thanks. The two toilets are 3 ft upstream from a sink that drains into the waste drain - can't I use the vent from the sink (2" for wet venting the toilets? I should also add that 3 feet upstream from the toilets a shower (with its own vent) empties into the waste drain.

my code (MA) would not allow it but yours may ?
at very least the shower vent would have to be 2 inch.

I may be able to move my toilets within 6 feet of the main stack - that way I do not need to vent the toilets, correct? How about the sink - is that wet vented thru the stack, or does it still need its own vent?

And using a double Y for the back to back toilets, I want to be clear that I can reduce the pipe to 2" past the Y and drain the shower toward the main stack, with a 2" vent between the shower trap and Y.

WA uses the UPC, like MA.

if there are no other toilets draining from above the point where the toilets drain into the main vertical stack then the toilets can be 9 feet away and will be considered stack vented.
mass plumbing code is its own unique code. we march to our own drummer.

There is another toilet draining into the main vertical stack from above, so I believe I have to be within 6 feet of the stack, right?

How about the sink that is between the toilets and the stack - is that also considered stack vented?

And using a double Y for the back to back toilets, I want to be sure that I can reduce the pipe to 2" thru the Y, and continue 3 ft to the shower trap, with a 2" vent between the shower trap and Y. Is that to UPC code?

I'm helping my son who lives in WA, but I'm from NH where we use the IPC, so it gets confusing for a non-professional. Thanks again.

if there is a toilet draining into that vertical stack then it can not be used as a vent no matter how close or far you place another bathroom piped in below it.
if you have a horizontal run and you put back to back toilets in it, the inspector MAY let you get away if there is a 2 inch vent upstream and a 2 inch vent downstream of the two toilets.
look at pages 12 14 and 15 here..

[www.kelso.gov]

Thank you Packy for the link to the code document. Lots of studying to do.

My favorite excerpt so far: 'Keep in mind that “stack vents” and “vent stacks” are distinct.'