i saw this in action yesterday -- one WTMD, and two xray machines. the left xray is fed by the true precheck line, and the right xray is fed by the regular TDC stations. i didn't really pay attention to how they were selecting people for it.

precheck had a steady flow, but not a crazy line. (6am flight tuesday morning) the merge didn't have a real impact on me, because the xray line i was in was all "real" precheck people. the line for the WTMD was 2-deep when i got to it, but that's a non-issue.