My kitchen sink and bathroom sink are back to
back. I was going to hook them both up to a
double sanitary tee until I found out I can't.
Can I stack 2 sanitary tees? I think I have
room for 2.

yes.
use a street san tee as the upper one to save room.
make the kitchen the lower of the two.

Thanks. Why make the kitchen lower? Because
it'll get a lot more use? I have room below:
If I lower the kitchen's, I'll have to cut
a new hole in the wall and cabinet and it'll
butt up against the hot water valve. The
bathroom wall isn't installed.
How does using a street as the upper save
room? I'd think using a street as the lower,
sticking it into the upper, would.

Unless you are installing them upside down, I don't know how you would insert a lower street tee into a higher one. The sink has a higher flow rate than a lavatory, so it can create "venturi" problems.

my rough in height for a kitchen is approx 14 inches off finish floor while a lav is approx 18 inches.
the top tee is a street and the lower is regular.

I would make the lower one street also and put a cleanout tee at the bottom.

I had a different idea about which end was
street. I was wrong. I've never seen one.
By venturi you mean the top sink's draining
could suck the water out of the lower's trap?
Unfortunately I have to go to a specialty
shop to get a street - and if I go to a
specialty shop I may as well buy a double-fixture.
Curiously the home stores carry double-fixtures
in ABS but not PVC.

I think what you might have seen and thought as a street was just a san tee with a 1-1/2" vent connection at top but with a 2" bottom hub and branch.

Yes the "major" fixture should drain to the lower san tee, your kitchen needs to be lower because it has a higher flow rate, and in addition, the flow may be pressurized either when a garbage disposer is turned on or when a dishwasher is discharging via a pump. Plus you may want a garbage disposer one day which again means a lower connection anyway.

My suggestion is to do it stacked, lower washer drain, higher lav drain. Use a double sanitary tee with street connection up top for the lav, the other side a clean out from the washer side. A regular San tee at the bottom for the washer. That way if you ever need to snake it you can open CO from washer side as the bath side is usually accessible (with a crowded vanity or a pedestal sink) and when finished snaking can turn on bathroom faucet to rinse the snake out as you retrieve it.