When i first got on the roofing forums about 10 years ago,
This is how all the other roofers said they did it.
They rediculed me when i said it was incorrect.
Everybody said they cut both sides of the hip.
I say you only cut one.
The reason why is because during sideways, blowing
rain water will actually travel up the roof and leak at the hip.
I overlap my asphalt saturated underlayment twice at the hip.
And shingles once.
That way if cap shingles blow off,
Or sidways blowing rain,
The hip remains water tight.