Suggestion 1:
If you have 2 different characters that must meet, let's say a blue one and a red one. Then place some objects that they must collect. The red character must collect all the red objects and avoid all the blue objects. and the blue character must collect all the blue objects and avoid all the red objects. Once all the objects are collected, then they can meet. If a red character walks over a blue object then they lose and must restart the level - same if a blue character walks over a red object.
Suggestion 2:
Add in more characters but they all still move together. You have 2 blue characters that must meet and 2 yellow characters that must meet. You win only when both teams are together.