This is intentional. Whether or not it should be changed, I don't know (I'd personally prefer the title card, zone name and all, to be displayed once/like in singleplayer instead of split onto both screens), but currently, the game makes sure that the player isn't playing splitscreen before drawing the act diamond.
Source: https://git.magicalgirl.moe/STJr/SRB2/blob/SRB2_release_2.2.6/src/st_stuff.c#L1384-1390
Closing this issue because it's confirmed to be intentional behavior.
closed