zahavasdad, restaurants might have more than one hashgacha because not every Rav agrees with or trusts every hashgacha and therefor their kehila does not abide by it and won’t eat in the restaurant. An owner who wants to bring in more business from more customers will try to please more Rabbonim.
On the other hand, there are neighborhoods where the VAAD, does NOT allow any hashgacha but theirs. So if an establishment wants the OU or Kaf K, the Rabbonim tell their kehilas NOT to eat there.