I don't know how to progress the drinks but to actually get to that I just followed wiki which was basically doing everything available story/event wise with each of them before, maxed their stats and date/sex each a bunch of times but here's wut the wiki says:
Prerequisites: Done Hang with Kleio at the Mall, done Morgan cinema date, Location Pub, Kleio and Morgan present.
Complete all of Morgans (current) story before talking to the girls about their relationship. Have 63 LP with both Morgan and Kleio, they each are 45 GP, and they each are >/= 25 KP then you can choose both of them. Otherwise, one or both of them will leave the game.