Who told you you could run on just one carb with this manifold.
Usually, 'running on the front (or back) carb' is referring to 3x2's where all the carbs are lined up on a common plenum area. On old 6x2's, on the street, the middle carb on each side was often just a dummy and the engine would run using only the corner carbs. This is possible because all the carbs are setting on the open plenum area that is feeding all the cylinders.
On my setup, I use progressive linkage, but the progression is between primary and secondary. The primaries open first and then the secondaries open later; but both sets of primaries open first, then both sets of secondaries (you need to know that my 'carbs' are actually 4 barrel throttle bodies, of which I have two, and the primaries are equal in size to the secondaries, they're basically just air-control valves). The linkage is the same though, whether we're talking throttle bodies or carbs.
I don't know if this'll work or not, but here's some views of what you're probably going for:
Pictures here