I believe that, logically, Sol would have approached Barzini first with his deal. Sol would have seen that Barzini--the would-be capo di tutti cappi in waiting--would have the most to gain from the money he'd make with Sol on drugs because it would help him push past Vito. Barzini probably told him that Vito would never make a deal if he knew that Barzi was Sol's partner, so he said, "Go to Tattaglia. He's a pimp--Vito's not afraid of him. Tell Tatt anything you want, but I'll be your silent partner."
As we've talked about before, I think it's more likely that Sol went to Tatt first. I'm returning to it just to post something from the book I never noticed before: Maranzano was an ally with Tattaglia who "controlled prostitution in the city and what there was of the thin drug traffic at the time." So Sol was in women and Tatt was in drugs - they seem the most natural of allies.
