Austin,
While the owners might be paying union dues and getting benefits, they are NOT union employee (as determined for plan purposes) subject to the union exclusion BECAUSE their employment is not subject to the terms of a collective bargaining agreement. That is obvious when you note their income level. This is key: YOU DO NOT GET TO COUNT THEM AS UNION EMPLOYEES FOR THE ERISA EXCLUSION. Just read the actual definition in your plan document; you will see the standard language of employees whose employment has been subject to good faith negotiations blah blah blah. That does NOT describe the business owners. The union does not set their compensation levels, their benefits, nor their working conditions. You need to be very careful here.
In fact, it is even possible that the union can renege on their retirement benefits some day if they get into a dispute with the union; the union will declare that they were never eligible for the retirement plan since they are not subject to the collective bargaining agreement (they most likely will return any money paid by the participants into the plan, but not the dues!).
You did not explain enough about the business; are there employees other than the owners? I assume so. You haven't told us what "YOUR PLAN" is or what it is you are trying to do. You mention "maxing them out in this plan" but it is not clear what "this plan" is.
But be VERY CAREFUL with these situations. We had some owners in New Hampshire who got burned; a labor lawyer suggested getting a separately written statement from the union saying that they understood that the owners were not subject to the CBA but, nonetheless, the union retirement benefits would not be revoked by the plan for that reason. Even that does not absolutely guarantee it, but it provides something called equitable estoppel if they have to go to court to enforce the benefit.
Larry