You can configure a CAN adres uniquely for each ESC in the "app" tab. For the master ESC (the one that receives the signal from the remote) I set the adres to 0, for the slave ESC I set the adres to 1.
Once the above settings are applied you can acces both esc's trough the single USB port on for example the master ESC (the one with adres 0):
To connect to the master ESC, uncheck "CAN Fwd" (default mode) and click connect -> you are now configuring the master ESC.
If you want to connect to the other ecs over the CAN simply set the box left to the "CAN Fwd" checkbox to 1 and then check the "CAN Fwd" box. BLDC tool should now give the green connected notification.
You can now switch between both esc's by checking and un-checking the "CAN Fwd" box. Be not to write the same app config to both esc's, then you'll have to reconfigure the CAN addres again to make them different again.