You might want to look at double doors for the opening. You can purchase them prehung. I would go to a lumberyard or a big box store and look at what they can order in. I purchased Jeld-wen solid mdf doors to replace doors in my house. I like the quality. I special ordered them a home depot.

Here is a link that might be useful: double prehung doors

The problem is that I have a tight stair well that I don't think I can get a prehung set of double doors around, unless I could take it apart first. But, I don't know if that's possible. I guess I might as well build the jamb and assemble the door myself at that point.

Has anyone seen any good online step-by-step guides for assembling a door jamb and hanging doors, I can't find anything.

Thanks for your help rjoh87646!

Before deciding on the doors, you need to make sure the opening will accept a standard sized double pre-hung. You would be better off taking the pre-hungs apart rather than hanging and setting jambs/doors if you have never hung a door, especially doubles. Another avenue is to get a finish carpenter in to hang the doors. If you wanted to trim yourself, fine, but at least have a pro set them if you go the hang/set the door/jamb route.

Like the last poster said make sure the opening is the proper width to fit a pre hung unit. Like he said prehung units that big are a lot easier to move with the doors removed. Mdf Doors wiegh a ton and 2 of them in the frame would be impossible to move. A frame can be jockeyed around a tight stairwell.

There is another route if the openings are a non standard size, bypass doors. All the closets in the bedrooms have them and I just ordered door blanks and installed them on new hardware. I recomend Johnson hardware for bypass doors.
It's cheaper to order from the company than have a big box order it in.

Here is a link that might be useful: johnson hardware