I know what your problem is, if you test the map in single player, then the second player becomes a computer, unless you already have another computer player.
You cannot play a map alone, 1 person only, because the warcraft system is based on "Victory". For Victory to be achieved you must have an opponant.
If you are making a multiplayer map, this should not be a problem, as long as you have one other player, the slots do not belong to anyone.
also, if you want to remove a player unless he is playing, you might want to do a trigger.
For example, if its like hero arena, and you want to only spawn a hero chooser for people that are there you might do something like...
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Spawn trigger
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Events- Time Elapsed is 5.00
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Conditions- Player 1 (Red) slot status equal to is playing
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Actions- Create 1 hero chooser at spawn area (reigon) facing defualt building degrees
Maybe not word for word, but i am doing this out of memory. Depending on your situation you can change it, the most important thing is that you know what the condition "Player slot status equal to is playing" is.
Also, if you make a slot a computer player, in the player options menu. Check the box saying "Fixed slots" in the top left corner where you add players. this makes it so that a computer player is locked into a slot.