After much experimentation we settled on a setup nearly identical to David's except we kept the original triple on the boom. This allows two alternatives: 4:1 gross/4:1 fine or 6:1/2:1. We tend to use the first in light air and the second in heavy, though I'll admit to laziness in changing from one to the other unless the conditions are extreme in either direction.
This might seem counterintuitive since the first has more power (16:1 vs12: 1) but there are other considerations. The real advantage of a 4:1 primary is in light air downwind when the line eases freely and can be trimmed rapidly. Despite being more powerful for heavy air, the 4:1/4:1 setup creates an unbalanced load on the triple block. In the second photo above, the left sheave carries the primary sheet, taking more load (4x??) than the two sheaves carrying the fine tune. In 25-30 knots where mainsheet loads are 600-900lb, this can exert enough torque to bind or deform the triple block.
There's also the issue of dumping the mainsheet. When a puff hits or you have to execute a duck it can be difficult to uncleat a fully loaded 4:1 coarse tune line and slow to have to ease the fine tune first.
Bear in mind two part mainsheets also have a propensity to tangle, requiring better cockpit housekeeping. We find it best if both cam cleats face in the same direction, as shown. It helps to drop the bitter end of primary mainsheet into the cockpit locker so it can't get knotted. Use a minimum length fine tune and be sure to use a different color line.
