I'm no MySensors guru to be sure. You started by saying you were considering a second Controller. Then mentioned two Gateways.
You didn't mention what particular Controller you are using now. I've been using Domoticz for my Controller for a couple of years. Domoticz is perfectly happy to talk to multiple Gateways. I have some sensors on a MySensors radio network exposed to Domoticz through a single Mysensors Gateway. I also expose multiple ESP8266 (wifi) sensor nodes, each of which Domoticz sees as a separate Mysensors Gateway. I also have several virtual gateways collecting online data from Netatmo sensors and other online sources.
You can certainly have more than one MySensors (radio) gateway if they are on different radio types (and I believe different channels). The concern is that Mysensors radio nodes must communicate exclusively with a single Mysensors radio gateway. You can't have a gateway wander around into the area covered by another gateway unless its on a different frequency, or routing may get all mixed up. It doesn't work like Wifi or cellular where connections are handed off.
Your sensors won't get exposed to Controllers at all. Just one gateway for any node and multiple Gateways should be OK...at least for Domoticz.
Hope I got that all right.