Your technique for the tenons will work but the critical positioning of the shoulder cut will be hard to get in exactly the same place on both sides of each stile. If the are not exact you will have a gap on one side. I read about a method that solves the problem and can say from personal experience that it works.
Basically you make a "sleeve" with an opening just tall enough to slide over the thickness of the stile and with the opening about a quarter inch wider than the stile on each side. The whole rectangular "sleeve" should be about 5 inches deep.
To use it , you slide it over the end of the stile and position it the same way you proposed with your edge guide, I.e. So that your router base rides against it at the right place to cut the shoulder. Then use wedges on the edge of the sleeve( remember you left a quarter inch space on each side). This will hold it plenty tight. Route one side, flip over board and all, route the other and you have perfectly matching shoulders on the faces of each long stile.