This might cover it,not sure.

4026.3 c
(c) Coupled entries. Should the same individual or legal entity required to be disclosed and registered as a nonmanaging owner for a horse be present in the ownership (including the managing ownership) of another horse or horses in a race, to the extent that there is a 25 percent commonality in ownership between any two of such horses, all such horses shall be coupled as an entry. In addition, the stewards may require the coupling of horses having a lesser degree of common ownership where the stewards deem it desirable in the public interest.