Here is a link to the thread that Tom mentioned.
http://festoolownersgroup.com/festool-tools-accessories/ts-saws-cut-great-stair-stringers/. Probably overkill in this situation. But if you decide to go the route I show, use 2 x 12's and rip them using a rail to make sure that the sides are parallel to each other and that they all match. Otherwise the method will create uneven stringers.
For pressure treated stringers, especially if a closed riser is required, I attach blocking to the inside of each stringer where the treads will be and fasten them to the stringer. I then fasten the treads to them. The stringers are carrying the load but when fastening I am not attaching to end grain which will over time split the stringer. Call me stupid, call me crazy, but it works.
Peter