If only signal put the same functionality into Android tablets

Why only tablets? Why not all devices?

For example, I'd like to be able to use Signal on both my main phone and my second phone using the main phone number.

I don't exclude what you want. I want what iPads without SIM have.

Second SIM bound to same identity isn't the same as non SIM adjunct device support.

Exactly. I don't even want that Signal knows about the second SIM, I want to be able to use it in another device, regardless if it has own SIM or not.

doesn't seem to be a difficult challenge for facebook.