It's been confirmed Halo Infinite will feature 2 player local split-screen and up to 4 players in online co-op for Campaign. After some reports stated otherwise 343 Industries head of design Jerry Hook confirmed it on Twitter and apologized for any confusion on the matter.
Hey everyone I saw some reports of campaign supporting 4 player Split screen for campaign and just want to correct. Halo Infinite will be supporting 2 player split screen and 4 player online co-op for campaign. Sorry about the confusion. #haloinfinte
— jerry hook (@hookscourt) July 25, 2020