Four teams are divided into two groups, which you will definitely do. In three cases, Party A and Party B are integrated. So the probability of them being together at first is 1/3. If they are not together, it is 2/3.

But if Party A wins the first game when they are not together, Party A can advance, and if Party B wins the first game when they are together, Party B can also advance, so that Party A and Party B will play the final together, with a probability of 2/3 × 1/2 × 1/6.

The total probability is 1/2.