FYI Google Fi has plans where you can get extra SIM cards that share the same data (data only, but all Google Fi calling is over data anyway, really; you can "call" from a computer).

I like the flexible plan because I'm almost always near wifi, and thus my bill is often just $35 or so, but the bill will never be over $80 even if I were to have my phone downloading 24/7. MVNO mostly on T-Mobile network. It seems that Google has a better MVNO deal than many others, less slowdowns.

Fi deprioritizes after 15 or 50 GB/month based on your plan, though. I mostly don't hit that, and even if I do, I've never really noticed it -- I'm in a low population density area.

See "Shareable data included for tablets and other devices" in https://fi.google.com/about/plans