Kim Zolciak and Cynthia Bailey are reuniting years after their initial run on "The Real Housewives of Atlanta" ... TMZ has learned they're starring in an upcoming Hulu series that could put a boatload of cash in their pockets. We're told Kim, Cynthia and their fellow castmates -- including Spencer Pratt -- arrived in Toronto this past weekend to begin 10 days of filmin… Our sources tell us Kim and Cynthia's 'RHOA' mini-reunion could have been even bigger ... because producers reached out to their former cast member NeNe Leakes and offered her a spot on the show, too. However, we're told she ultimately turned down the offer. Hulu announced the show back in September, but the streaming service didn't reveal who would make up the cast ... and now we know at least 3 of the bigger names. The premise of 'GTGO' ... contestants live in a mansion for 10 days and battle for a pool of up to $1 million in prize money. The pot rises $1 every second -- there are 864,000 seconds in 10 days -- and players work together on challenges to boost the prize money up to $1 mil. Contestants can either stick together and split the cash at the end, or take the money and run.