How to Measure a Magnetic Field with 2 Molecules of Unknown, but Opposite Spin

You have 2 molecules that are entangled such that they must have opposite spin. However, you do not know which one has the positive spin and which has the negative spin. Both molecules are bound to a surface. By rotating this surface, how can you determine the direction of the local magnetic field?

I know that this problem can be solved because it has been suggested that birds do exactly this in order to help migrate. I do not see how this is possible though. Both molecules experience forces in opposite directions, so without knowledge of the molecule’s spin you cannot determine which force is toward the North Pole and which is toward the South Pole. What am I missing?