Is it possible to pair one Pokémon GO Plus + with multiple Pokémon Sleep accounts, or more than one Pokémon GO Plus + with one Pokémon Sleep account?

No. A Pokémon GO Plus + can be paired with only one Pokémon Sleep account at a time, and a Pokémon Sleep account can be paired with only one Pokémon GO Plus + at a time.

Please visit here to learn more about unpairing the two.