I know this might not be an option for you, but I have used Home Assistant for this purpose. Simply install a HA instance and enroll both of your handsets with the companion app.
The challenge with apps like this is, that they need some kind of server by definition.
Let me know what you find.
Look into Zood (Android only), OwnTracks (iOS and Android), and Grid (iOS and Android)
Also recommend using these resources to find alternative apps, services, and platforms:
Another plus for zood, I dont know if its still being developed but it works well.
Thank you, I'll have a look!
I'm not sure if you've heard of Paralino, but I've been using it and I've been happy with it so far.
Unfortunately, it's not open-source, but it's more privacy-oriented, end-to-end encrypted, can be installed with an APK, and subscriptions can be bought outside of the app stores.
It's not exactly the perfect solution that you're asking for, but it's a huge step up from Life360.
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