For OTAA, even if both networks would accept the Join Request, the gateway can only transmit the Join Accept from one of them. That Join Accept defines the random session secrets (NwkSKey and AppSKey) and the DevAddr (with the network-specific prefix like @UdLoRa already noted). The networks won’t know each other’s randomly generated secrets, and won’t know which DevAddr was assigned either (if it would ignore the prefix).
Also, for downlinks a gateway needs to adhere to duty cycle regulations, which one, and only one, server should control; see Can the TTN Kickstarter gateway forward to alternative or multiple backend servers?
So: what you want won’t work.