If I buy Don't Starve from Humble Store, do I get Don't Starve Together too?

Get a coupon for steam directly... They're cheap to be had afair. Also note that steam refunds work only if bought at steam not through a keyseller. Imho thats the gravedigger for 3rd parties unless they are substantially cheaper around the clock.

You only get Don't Starve Together if you bought it before June 4, 2015, even for unredeemed copies.