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Google Messages dual SIM support for RCS rolls onto US phones, per reports
It looks like it's still taking some time, so you might still have to wait a while.
Users have taken to Reddit to report an apparent stable, server-side rollout of dual SIM support for RCS chats in Google Messages. Users have taken to Reddit to report that Google Messages has started pushing its dual SIM for RCS support to stable users/versions (via Android Police). The post adds that reports from other users say they've received proper support with SIMs from T-Mobile, AT&T, and Google Fi in conjunction with those previously mentioned.
Or read this on Android Central