When multiple push or pull effects apply simultaneously, they are combined by adding their values together. For example, if an ability with "Push 1" gains "Push 2," the result is a "Push 3" ability. The same applies to pull effects.

• The combined effect is resolved as a single push or pull with the total value.
• The direction and distance of the movement are determined as usual for push or pull.

This is stated on page 32:

Multiple push or pull effects are combined. For example, if an ability with “Push 1” gains “Push 2,” the result is a “Push 3” ability.

Source: page 32.

Source: Core Rulebook (pp. 26, 27, 32, 46, 76)