It has to be shown that they are sufficiently independent whatever that means.

From RB Salzburg's wiki:
https://en.wikipedia.org/wiki/FC_Red_Bull_Salzburg#International_competition_history

...this raised the issue of a possible conflict of interest between the clubs due to the level of influence exerted by Red Bull over both teams and the close sporting relationship between them in various aspects. After examining the operational structures during June 2017, UEFA declared themselves satisfied under their regulations that the two clubs (particularly Salzburg) were suitably independent from the Red Bull corporation, and sufficiently distinct from one another, for both to be admitted...