When two controllers connected, player2 input is disabled in full screen mode in PIE.
This doesn't appear to be in focus. If seeing the any focusing widget, player1 focused on SViewport, but player2 focused on SWindow and SOverlay.
Input is disabled in full screen mode only. Reselecting the viewport again allows it to work correctly, but only the first time the problem occurs. It worked correctly in 4.23.
Workaround for UE5.1:
The character doesn't move when input from controller 2.
The first controller input is allowed.
Both controllers input is allowed.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-94984 in the post.