If I have pairs of window casings (head & sill casing or side casings), one method I use to cut two pieces that are the same dimensions is to make both miter cut on the first one and just cut one end of the second piece.
Then I put them both on edge with the inside of the casing up. Then I line up the two ends and mark the other end of the piece that still needs to have a miter cut.
I think this takes less time and is more accurate than measuring both pieces. If I have lots of cuts of the same size, then setting up a stop is a good idea…but for just cutting pairs of matched moldings, this is what I usually do.