If each person has 2, and each party is smart enough to only run 2 candidates, then the party with the most voters will get both slots.
But if each person can only vote for one at large position, then the party with an outright majority can't guarantee winning both positions.
In this form of proportional voting it is absolutely critical that either each person only have one vote, or that each person can vote multiple times for the same candidate. Otherwise the dynamics change a lot.
Cheers,
Ben