This is a dead valley. Roofing shingles don’t belong here.
Instead, single-ply membranes such as EPDM, PVC, and TPO with a separation layer between the shingles. Or a 2 ply SBS Torch-on membrane can be used.
Depending on your location, a 3′ minimum upturn is required in the chance of a snow load.
If any type of repairs or replacement is needed in the dead valley on your roof, professional roofers must be contacted.