This was brought up many, many times in the past, but here goes yet another try.
Nabu Casa can only support one web-connected instance per account. The Home Assistant app (at least, the iOS version) supports multiple hosts, but Nabu Casa does not. Managing more than one site (I have three, personally) becomes a nightmare of either signing out/in all the time (which also defeats any automations related to the phone itself, btw) or deploying VPN tunneling technologies (which defeat one of the principal reasons for using Nabu Casa in the first place).
I understand there are developer and security challenges to this. I also understand that the funding model would/should change. But I cannot contribute to NC until this is fixed, because NC is useless to me otherwise. I instead contribute directly to creator Patreons as a means of showing my appreciation for all the hard work on HA.
Please reconsider your stance on this feature and begin its incorporation at some point.