Using family set up is an option - but not a good one.

Personally in this situation I’d be inclined to use two lines on the phone itself and then you’ve got the solid connection throughout.

I don’t think WiFi calling works using the other lines data connection so it might be a case of setting up call divert from your main number when unavailable to the alternative line.

When you’re out of cell coverage for your main line you’d not get SMS messages but iMessage would still work as expected for all numbers.