Have you noticed a change in how they hang recently? Is one different than the other?
The most common orientation is one in which the long axis of the testicle is roughly aligned with the long axis of your body (vertical). However, about 10-15% of men have a "bell clapper" deformity in which one or both of the testicles hang in transverse direction (horizontal). This is something that a person is born with and results from fewer attachments of the testicle to the tunica vaginalis, which covers it.
This "deformity" in itself is harmless, but it does increase the risk for testicular torsion, a condition in which the spermatic cord twists and cuts off blood flow to the testicle. It's an extremely painful condtion that requires treatment in <6 hours to save the testicle.