If I can't figure out how to use two horn cleats, my backup would be to use a fiddle block and camcleats like this:
http://l-36.com/3161fiddle.gif
If I use a double block like in Harken 401 or 402, then the lines would cross as the block swivels as the boom block is a fiddle block.
This would keep everything in line although it would require that I fabricate the base for the camcleats. One of the posters suggested always using fiddle blocks and not doubles or tripples. As I already have enough fiddle blocks to do this, that is an added advantage. I do think, however, that two doubles and a single mounted below the boom double runs true with the single mounted crosswise.
I have not figured out how to eliminate the turning blocks and use just horn cleats in a way that seems like it would be workable in race or emergency conditions. This is still my goal.
Allen