EPDM is best stand-alone and not joined to a felted roof. This isn't the way they tell you to do it on the EPDM course, but its what the lads do.
As long as you have done a perfect clean straight slice between the two roofs, before you lay your board down 1st glue some rubber down overlapping a foot under and foot over, then screw the board down tight, screwing well near the edge. Your board must be butted hard against the straight cut felt.
Run a bead of leadmate or lead sealant between the wrapped around rubber and the felt edge. Let it dry whilst u board the rest of the roof. Don't use silicone!
Then you lay the rubber as normal overlapping onto the felt slightly and then seal it again with lead mate.
Finally you then put a length of epdm seam tape over the join and then onto the other roof, use water based glue as the solvent stuff reacts with the oils in the felt. the other roof has to be as clean and dry as poss.
You shouldnt really join rubber to felt, its an absolute pain and only do it if the join is at the highest point on the fall.
See my excellent drawing, lol (which doesnt show final seam tape join)