In the nonideal solution, why can't one of the solute be positive deviation and the other be negative deviation?
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.