From my experience in Australia with an iPhone XR running iOS 16, I used SIM 1 with Kogan NZ and SIM 2 with Optus AU.
I set cellular data to SIM 2.
When connected to WiFi, SIM 1 used WiFi calling and the carrier name changed to WiFi Calling.
When disconnected from WiFi, SIM 1's call and text used SIM 2's cellular data and the carrier name of SIM 1 changed to Using Mobile Data.
Neither option triggered roaming fees, and I was able to make/receive calls and texts to NZ/AU for free, just as if I were in NZ.
If I turned off SIM 2's cellular data or removed SIM 2, and disconnected from WiFi, SIM 1 would roam on the local network.
