McFeely: We're many drafts in.
Markus: We're on the third draft of movie one and the second draft of movie two.
McFeely: It's going pretty well.
Polowy: So you're actually writing them in tandem?
McFeely: As much as we can. Yeah. Literally on any one day you're trying to write only one, but that doesn't mean the next week, you're not working on the other one. And certainly, notes are coming in, and all sorts of production stuff is coming in. Like, 'That thing over there is now a window, not a door. Let's adjust that.' Or, 'We're not going to that town; We're going to this town. Let's adjust it.' That kind of stuff happens constantly, and will for the next-.
Polowy then asked the screenwriters about how they narrowed down the number of characters that will actually appear in those films.
McFeely: Our deal was, when we lock ourselves in a room with the Russo brothers, and a couple of execs from Marvel, one wall was movie one cards and one wall was movie two, and one wall was little baseball cards with every character still alive in the Marvel universe.
Markus: Down to Darcy's boyfriend from Thor 2. Like just anyone who did not die and has been established as alive, and then went- [pretends to move cards around on a board].

Avengers: Infinity War Screenwriters Offer Script Status Update
http://comicbook.com/marvel/2016/07/31/avengers-infnity-war-writers-offer-script-status-update/