Comment by jcul

Yeah, would like this too.

Whatsapp added this recently and it is very convenient. You can link a companion device in the same manner you sign into WhatsApp web.

A kind of hacky workaround (that I used to use for both signal, WhatsApp and others) is to set up a server with matrix bridges running and bridge your signal, WhatsApp etc. so then you can install the one matrix client on all your devices.

But as most apps do support multiple devices these days, bar signal, it doesn't feel like it's worth the effort. And I seem to remember the signal bridge in particular being a little buggy.