It is possible that the cross-checks on your lines are set too far back bringing the horses, especially their heads, together. If this is the case, you can correct this lateral misalignment by setting the cross-checks further up on your main lines. If you bring them too far forward, their heads will be carried toward the outside. Try the change incrementally to see how they do.
Another possible cause is that the centers on your neck yoke and evener are not the same. For example, if the center on your neck yoke and evener are 36″ and 40″ respectively, this would cause the horses to lean into each other.
George
.
