Unfortunately in that case, the CL spot won't be taken from Milan. Simply there will be 5 Italian teams in the CL next season.
In the scenario you mentioned, two clubs from the same country have to win CL and europa league, and both have to finish outside the top 4 in their donestic league. In that situation, they take the spot from the 4th team because the maximum number of teams that a country can have in the champions league is 5. For example if Roma and us miss the top 4 but we both win the CL and EL, then they take the spot from Milan if they finish 4th in the league.