A bit funny, but in the latest Spotify Android app the changelog talks about the possibility to use 2 devices to use Spotify Connect with your speakers. Somehow they call it a new feature, but it worked like this for some time now. Here’s the changelog:
New: Want to use Spotify Connect on any speakers? Plug a phone or tablet into both a charger and your speakers. Log in to the same Spotify account on another device, and use it as a remote.