I overlap them just a bit, pin on each side of the diagonal (place them parallel to the diagonal and far enough from it so they won't get sewn over), then lay an index card along the diagonal and mark it. The index card is flexible enough that it won't wobble when you lay it over the pins like a ruler will, but the edge is sturdy enough that you can mark a nice straight line with it. Then sew along the mark, leaving the pins in place until done. Works well for me.
Sometimes there is a little bit of discrepancy but as long as it's smaller than your seam allowance it won't show.
Rob
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