When will the playtest rules, and with it, the first playtest adventure, be available on demiplane?
Secondly, are there people interested in playing it? As a dedicated DM, I would love to run those adventures, and provide feedback to Paizo based on our games.
We plan to launch when the playtest begins at Gencon! That will include the playtest rulebook, character tools/compendium, and the first playtest adventure. We’ll shout from the rooftops the moment everything is available. 
Where starfinder?
Its live on paizo, when on demiplane? ETA?
We’re working on the release process currently. Your patience will be rewarded!
You target my one and only weakness
Slightly tangential and probably too early to get an answer but are there plans of having the two 2e Nexuses combined/combinable once the playtest is finished and the official release is published? It would make sense with all the advertising about the compatibility of the two 2e systems that we would be able to, for example, create a character with a Starfinder species and a Pathfinder class.
They are indeed!
You can turn on content for the other game inside the “Rules and Preferences” section found under the character portraits in either builder. 
Woohoo!! Awesome! Thank you so much for the quick response and good news!