It happens automatically You'd probably want to manually select a network for your main provider (T-Mobile in my case) that they don't support, to make sure you have no service. Otherwise it'll latch on to one of their roaming partners and not use the second SIM to simulate Wi-Fi.

I think this feature really makes traveling internationally so much better. Even with the free 256kbps roaming on T-Mobile, it'd be better to get a local data-only SIM and get high speed data much cheaper and still make free US calls/texts on your original T-Mobile number. Best of both worlds