If they are in exactly opposite directions then they must be equal to give zero resultant. 
If they are not in exactly opposite directions then one of them could be resolved into two directions leaving a resultant. 
eg: One vector is horizontal and a second one acting at an angle to it in an opposite direction. It could be possible that the second would have a horizontal component equal and opposite to the first, but the second would still have an unbalanced vertical component making equilibrium impossible.