2 companies are not controlled group nor are they asgroup.
If someone has ownership in each,what prevents him from having 2 db plans,both with max benefits?

Nothing.

Depends upon the extent of ownership. Refer to IRC 415(h).

thanks

Sorry to mislead. For some reason I read into the first post that the CG determination already took into account 415(h). Clearly this was in error.

415 looks at all plans of the employer, not the employee.