In the nonideal solution, why can't one of the solute be positive deviation and the other be negative deviation? In the nonideal solution, why can't one of the solute be positive deviation and the other be negative deviation?
Let's say A and B makes a nonideal solution.
The moleculat force between A and B means A-B.
Let's say A-A > A-B > B-B.
Then A would evaporate more than in the pure state, and B would evaporate less than in the pure state.
For multiple component systems we use activity coefficient $\gamma$ in modified Roult's law as:
$$K_i=\gamma \frac{P_i^\star}P $$

and $\gamma$ is defined in the terms of $f$ : fugacity or fictitious pressure for a real gas.

You should read further about "Vapour Liquid Equilibrium" for multiple components.

Maybe:  http://www.journalrepository.org/media/journals/BJAST_5/2012/Dec/1355741742-Olaleke312012BJAST1789.pdf. You can see there is no reason why it can't have deviation in multiple directions.