Post Election Edit Fast Formula when an employee and spouse working in same organization

Please suggest a post election edit fast formula when an employee and spouse/ dependent partner working in the same organization.

It should validate that spouse and employee are not covered in the same plan.

Example:

If Employee1 and Employee2 working in the same Organization.

Employee1 have covered Employee2 as spouse for 'MED1 plan'.

When Employee2 try to enroll in the 'MED1 plan'. It should throw an error message.

But when Employee1 alone enrolls in 'MED1 plan', Employee2 alone can also enroll in the plan.

In any which way they should be covered only once in this plan.

This needs to be applied for all the plans in the program.