I tried using two of the AC1200 range extenders in a 2200 sqr foot house and there's been an almost constant conflict between them.  If I reset all wifi, including main router, they work well for a little while.  At some point in time though, I start notcing them seemingly cancelling each other out.  My devices have a hard time connecting to the network and neither extender appears in the "My Devices" tab in the tp-link app.  If I go down to just one, it works fine by itself, but no devices ever connect to it.  I'm sure the signal is weak in the corner of the house farthest away from the main router though, because I lose wifi bars.  I guess these things are really that weak and useless...

How did it work for you, incidentally?

Usually with extenders you want to have a different wireless name from the router. In the case of having two extenders so three devices being the same network name will cause the client device to constantly jump to those three. For stationary devices the setup sould work fine as they would stay connected to that one extender, but for devices that will roam, you will likely see constant internet interruptions as it jump from one point to another.