Rocket league won't let me play as player 1 using my steam controller. It says "Press |> to join" and then goes into split screen mode.
Disable desktop mode for the steam controller using the following steps:
Steam > View > Settings > Controller > Desktop Configuration > Browse Configs > Show Other Controller Types
Choose one of the disabled ones (PS4 or Xbox) and apply it.
This will disable the mouse/keyboard emulation of the steam controller which apparently confuses Rocket League.
(found via firstname.lastname@example.org)