Can I create a "chat" app between two Bangle.js 2 watches? Can I use Bangle.js 2 with Puck.js ?

Sorry for the delay - I was off last week and didn't get a chance to follow this up.

But yes, 'show nearby' uses the long range Bluetooth so should work quite well. Long range is a bit strange in it can either be an advertisement (easy) or it gets negotiated for a connection if both devices support it. I'm not 100% sure when it get used and I don't think we can force it onthe Bangle yet.

... so the long range advertising would be the best bet. The show nearby app could be modified to (rather than using GPS) advertise a line of text, and then the listening device could scan and display that when it changed - I'd be happy to help with an example if you want, but chat should work fine.

Puck.js v2 - can I combine it with bangle.js 2?

Yes, absolutely! using the Puck with Bangle.js is pretty much the same code as if you were talking from a Bangle to a Bangle.

What could be the battery life of Puck for reading the temperature let's say every 1 minute and transmit it?

This basically does that already: https://espruino.github.io/EspruinoApps/?id=bletemperature

And power usage should be around 25uA on average I think, so the battery should last just under a year. If you reduced the advertising interval you could increase that substantially though.