I don't know, but (at least some of) the apps that need multiple logins for precisely the scenario you suggest, seem to have solved this. Here in India we have some grocery apps that deliver (mainly perishables) every morning, such as Doodhwala (https://play.google.com/store/apps/details?id=com.bangertech...) and MilkBasket (https://play.google.com/store/apps/details?id=com.milkbasket...).

So I register with my phone number, I get an OTP (via SMS) and login into the system to add stuff to my basket, etc. My wife also logs in from her phone, but rather than registering afresh, uses my phone number. Now I get another OTP which she uses to login from her phone. That's it. Same account is logged into on two different phones and the login persists.

I don't know if this is by accident or design, but it works. Hopefully it'll continue to work, and they don't try to "fix" it because it wasn't meant to be that way...