If you were representing the union before, it will be a risk to pick a side after the breakup of the union. It likely can be done, will no doubt require extra time to make sure one is not violating privacy of the union, or either former member of the union. Might be a case where taking on an extra 10-15% for the retained client, at least for the first year or two, may be needed to cover your extra time. IRC, I have seen here at least some refuse to represent either after breakup of the union, because of the risk. I can see where it would not be uncommon to have to provide records for both parties, and even the one you elect to retain getting mad at :"helping" the other side.
I get something similar when a business is sold, and the new owner and former owner do not make a proper break in the employee records. Many believe the new owner should have access to and responsibility for the employees for the full year. The proper method is the former employer terminates all employees, and the new owner hires any employees they desire, even if they are former employees of the prior owner. All too often, the former owner provides employee data to the new owner, which is never allowed (although there may be specific exceptions I suppose). I had to fire myself and rehire, after incorporating. I had to make sure I collected I9, W4, etc, from myself as a new hire.