It is often said that when using two carabiners at the same point, such as an anchor, they should be opposite and opposed.
What exactly does this mean?
This is what a diagram of opposite and opposed carabiners looks like.
The gates are on opposite sides and form a X when laid over the top of each other.
The purpose of doing this is to make it difficult for both gates to open at the same time, because even if the rope doesn't slip out, a carabiner with the gate open is much weaker.
Notice that even if one of the carabiners were to shuffle around (when the system is slack) so that the gates end up on the same side, then they will hinge at opposite ends, minimising the chance of simultaneous opening:
If one were to have the gates on the opposite sides opening in opposite directions, vertically flipping one of the carabiners in the first image like this
The problem is that a rotation will bring both gates to the same side and opening in the same direction. Notice how the gates are on opposite sides, but when opened they don't form an X