Didn't you select the two doors at once when you made them into func_door accidentally? Easy to check: select one of the doors. If the other door becomes selected too, then you made ONE door only in real.
In this case press right mouse button, select Ungroup Entity, then remake the doors one by one.
Or perhaps your doors have the same targetname so the trigger opens both of them.
Normally a door will move as many units as it's width in the direction of the movement. So if your door is 100 units wide then it will move 100 units when opens. The lip value tells the door how many grid units it has to "leave visible". The defualt lip value is 8, so the door will move only 100-8=92 units.
If you give the door a bigger lip value it will move less. If you give it a lip value bigger than the width of the door then it will move towards the opposite direction. You can use negative values for lip of course.
Playing with lip makes the func_door more powerful than it is for the first sight. You can make even elevators and horizontally moving platforms with func_doors if you want.
Btw. I think this is the forum where you will get answer for almost any (including noob) question.