According to the official FAQ, Halo: Campaign Evolved on PS5 will require both players in split-screen co-op to have their own PlayStation Network accounts, active PlayStation Plus subscriptions, and linked Microsoft accounts.
Normally local split-screen games normally don’t require PlayStation Plus.
The FAQ also states that Xbox players only need separate Microsoft accounts for split-screen.
If these requirements are accurate, PS5 players will need two PlayStation Plus subscriptions just to play couch co-op, in addition to two Microsoft accounts.