@Zonelab 

Absolutely possible, I do a fair bit more than two using the same TPlink cloud account.  When you set up new controllers, just register to the cloud with your single TPlink ID.  They will appear as a list both via the webpage and the app.  Super useful.

In theory you could register a subset of switches/APs at one site as a different logical 'site', so long as the controller is not in the same broadcast domain as the other controller, they won't conflict.