In a commercial kitchen, we currently have four 5-ton heat pumps controlled by a Trane XT500C stat. A series of relays is used for the fan, compr, and rev valve functions. Under the current setup, all four units turn on at the same time. We would like to set this up as a 2-stage system, where two units turn on and a second set turn on if the first two cannot keep up with the demand. I thought that this could be done via the Y1 and Y2 connectors on the stat. This apparently is not the case. The Y1 circuit applies 24V (for the compressors) as it should. The Y2 connector appears to maintain 24V at all times. Any suggestions?