If a creature has two regeneration shields on it — one from a Debt of Loyalty you cast and one from another effect — and it would be destroyed, its controller chooses which regeneration shield to use. If that player chooses to use the Debt of Loyalty shield, you’ll gain control of the creature. If the player chooses to use the other shield, you won’t gain control of the creature, but the Debt of Loyalty shield will remain on that creature for the rest of the turn (in case it would be destroyed again).