Father owns 100% of company. Will be gifting shares to son-in-law. They may be starting another company. Daughter is not an employee.

If the ownership of the companies were structured that there would not be controlling interest -- either father owns 100% of company A and son-in-law owns 100% of company B; or each owns 50/50, is there control?

I know there isn't attribution to each other (no double attribution). But would the daughter end up indirectly owning 100% of each and therefore there is control?
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Never mind -- I forgot the one essential fact that makes this a non controlled situation. The daughter is not minority aged so the father's stock would not be attributable to her. So no control unless either the father or the son-in-law has controlling interest in both companies...

That last statement is not entirely correct. For example, the father could own 51% in both companies and it still wouldn't be a controlled group.

Assuming it's only the son-in-law and father with ownership, why is that so?

It would be so. I should have said, it wouldn't necessarily be a controlled group. Father owns 51% of A and B, friend Bob owns 49% of A, and son-in-law owns 49% of B = no controlled group, yet the father has controlling interest of both companies.