Combining vents in attic

I am combining some vents in our attic. I have a 1-1/2" line teeing (horizontal) into a 2" line. Can I use a sanitary tee (horizontal) or should I use a combo wye for this vent?

Thanks!

Vents can use any fitting you want to, just remember that if you use sanitary fittings in the vent, inspectors will want to see the air flow going the right way even though it is immaterial.

Do horizontal vent lines need 1/4" per foot of slope as well?

Yes, your vent lines need the proper slope. Rain, condensation, or whatever may get into the pipe, and it must be allowed to flow down and out. Depending on how many different vent stacks you are combining, you may need to increase the size of the pipe going up from there. Depending on where you live, there's often a minimum size penetrating the roof to minimize the chances of it closing off from hoar frost in the winter.

Vents need "some" slope. You don't necessarily have to go with 1/4 IPC though. Sanitary tees are not vent tees but then again vent tees are hard to find. Though an inspector may question the "air flow" there's nothing in the code that says San-tees used for vent have to be installed in any particular direction. Air is going to flow regardless of the placement.