You will probably have to do a Split-Face on the riser pipe. This will give a funny result at the points on the riser pipe.
Your Split-Face line will start at the end-point of the cope of the Riser pipe.
Did you ever figure this out?
I am dealing with exactly the same problem, putting weldments on two pipes of same diameter. I tried to use Split-Face but couln't get this to work. Help would be very much appreciated.