tl;dr If you have Xfinity Mobile and want to go dual SIM -- using your Xfinity Mobile SIM for calling and texting, but a different SIM for data, you should turn off Global Travel Pass in addition to configuring your phone settings appropriately.
Longer version:
We spent a month in Germany in 2024, setting up dual SIMs on our iPhones as follows:
Kept our Xfinity Mobile SIMs active - so that we could get texts from home if necessary - but with data roaming turned off.
Installed Airalo eSIMs which we set as our Cellular Data defaults.
Turned off Cellular Data Switching.
Added Global Travel Pass on our Xfinity Mobile lines, which (at that time) would cost $10/day only if we made or answered a voice call, sent a text, or used our Xfinity Mobile lines for data.
We were unpleasantly surprised when we returned home and found that we had each been charged 7 days worth of Global Travel Pass ($140 total), despite no texting or calling on those days, and with data settings as above.
Turns out that even when the phone settings point to a different SIM for data, and data switching and data roaming are off, the phone still sometimes generates very small amounts of data traffic - maybe 0.2Mb/day - on the Xfinity Mobile line. Not sure if this is an iPhone thing or an Xfinity thing. It's enough to trigger Global Travel Pass. So we paid $10 for 0.2Mb of data, for several days. :-(
So for our trip to France last fall, we again bought Airalo eSims ($16 apiece for 10GB for 30 days) and configured our data settings as above. BUT we also turned off Global Travel Pass on our Xfinity Mobile lines.
This worked great.
Xfinity Mobile no longer charges extra for calling and texting while abroad, so that worked as it does at home.
Airalo data worked fine.
Our phones still generated those tiny amounts of data traffic on our Xfinity Mobile lines, but with Global Travel Pass turned off, we were charged only the pay-as-you-go rate of 30 cents/MB, which came to only about $5 between the two of us for the 4 week trip.