A regular coupling has a stop ridge in the center of the coupling on the inside to insure both pipes are inserted to the proper depth.

What you need is a "Repair Coupling" which has no internal stop ridge. The repair coupling is slid onto one piece of pipe, then when both pipes are aligned you apply glue to the pipe at the joint area and slid the repair coupling across the joint and position it over the joint.

Thanks. I just did a quick search and read where some people had troubles getting a leak-proof fit. Are they hard to work with? Should I just use a Fernco band instead?

How about my 2nd question? Is a 5 ft toilet drain unvented OK?

Thanks

The local hardware store has "Heavy Duty" PVC cement that I have found to have a slightly longer open time than does the medium duty. It gives a little more time to slip the coupling into place. Practice a couple of times dry to be sure you have the technique down-pat, then apply primer and plenty of cement as quickly as possible and slide the coupling. Be fast!

If you have not used repair couplings you can mark the coupling in the middle (sharpie or el marko) and then measure from a convenient point (underside of floor, bottom edge of joist, etc.) to the end of the pipe to be joined.
After putting cement on the pipe slide the coupling into position based on the measurement to the mark ion the coupler.
The longer set glues work better here, but the repair coupling (AKA 'slip coupling') is not tapered like a regular PVC fitting.
A Fernco is easier to use but costs more.
For a single repair it is easier to use.