A customer called in while installing a Model 1000 to add an additional control wire to the condenser cable. He was using the two transformer method to install and wanted to confirm that the transformers were in phase.
To do this, we utilize the existing two wire condenser cable and hook them to R and C of the indoor transformer. We take the one connected to the indoor C and attach that to the external transformer C.
We then measure the voltage between the R coming from inside and the R on the external transformer. This should measure ~0V. If you are reading a voltage, swap the leads on the external transformer.