Occupying the Same Square
Two or more human sized characters can occupy the same 5' square. However they may impinge on each other's ability to act depending on the type of action being attempted.
Generally, attacking someone from a square with multiple occupants adds a 1d6 situation bonus to the defender. This is because the attacker cannot swing their sword as easily or is more likely to be nudged whilst trying to aim a gun. In the reverse situation, when two defenders occupy the same square, there is no bonus. A character's disadvantages from being restricted are offset by the extra cover earned from their companion.
If a square is occupied by two characters attacking each other there are no situation bonuses as both parties are equally advantaged or disadvantaged.
Other types of actions, such as awareness, are unlikely to be disadvantaged by multiple occupants; however each situation should be judged on its own merits. The nature of the action, the number and size of the occupants and the general environment are all factors to be considered.