Absolutely. You could have one player control the party and the other the player character. I'd say that experimenting with 2 player characters would also work, using the same difficulty modifiers as you would for companions.
Scaling for enemies is already listed on page 24 (just add +1 Health for each player past the first).
For handling bosses, increase the Health of the boss by +5 for each player above the first, i.e., 2 players = +5, 3 players = +10. This is a distilled version of how Souls' games adjust difficulty with summons and would honestly work here, for the most part.