I would definitely consult a lawyer on this. I would imagine that one of the issue becomes if you own a union company and non-union company that can service the same customer / project and you "divert work" from the union company towards the non-union company. I imagine the recommendation would be to keep operations completely separate so that the union cannot claim the two entities are really one combined entity and should be subject to the same collective bargaining agreement. One thing you may want to consider is making the non-union company a union company where you define the rates and work rules, so you have a net equivalent cost structure but now both are union.

I only have experience on the corporate side, not as an owner, but the CBA should have very explicit call outs on what is part of the business is covered, and likely geography. In my experience there is also quite a bit of effort into making sure there is an "airgap" between the groups so they don't work together. Don't leave any gray area and document everything.