Any relationship is possible. These teams can be owned by same person, are friends, or one of them is 3rd party who wanted to help another team indirectly.
In addition, teams owned by same manager not necessarily manipulate results between his/her team. Twenty seasons ago, both my teams were involved in a 3 team race for league champion at level 2. The worse possible result for match between my teams were a draw because both teams would loss 2 points. But I didn't want to manipulate results. So I used backup players in these matches and let the match ran normally. Eventually, at least one of these matches was a draw. Fortunately, one of my team was crowned league champion but the other just finished 3rd.
After that season, these teams were not drawn to same league again even if they were always promoted to same level. After 7 or 8 more season, I gave up one of them as it finished 7th.