Yes, you can only use one fixed plate. There are two different ways to do it.
1) Mount the plate to the spoilboard or table, and set Z zero to be the bottom of your stock in Aspire.
2) If you zero to the top, then use the Material Offset option in the 2010 Screenset to specify where Z zero is relative to the bottom of the fixed plate.
With either method, you'll need to jog over to the fixed plate, and run the Initial Auto Zero, which sets Z zero, and stores the fixed plate location.
The after tool changes, the tool will automatically be measured to the fixed plate.