The trouble is the metal roof's profile. It would be easier if it were smooth (flat).
My instinct tell's me that it would be best to tuck a strip of 30 weight felt or rolled roofing under each roof rake (edge). Starting at the bottom by sliding 4 inches of the underlayment (felt) under the asphalt shingles and the metal roof. This will close the gap at the seam of the (2) roofs. Overlap each row by 2-4 inches.
Next you can either extend the metal roof onto the asphalt roof (if you have some). Overlap each roof with 24 inch length's and secure it as normal to the metal roof.
Or you can simply glob some roof patch down the length of the roof seam and press in a fiberglass mat followed by either metal flashing that is 4 inches past the seam on either roof (and nailed/screwed to each roof) or a strip of mineral roofing (rolled roofing).
The rolled roofing can be adhered to both roofs with rolled roofing cement which essentially glues it to the roofs surface. It should be nailed also.