But virtualy it is one ecu not two. An ECU has one map to control the EFI. When you have a piggy back you have two maps. The piggy back map is controlling the inputs to the ECU's maps. The ECU's maps gives the final instruction set. A/F controller is more or less a piggy back to the main ecu.
On the Zem there is one map only which sits in the main slot where the stock chip use to be. You can't have a Zem piggy back on top of a stock chip or JWT chip. The Zem replaces the main chip (main map) .