Neither Bless is "more potent"
Therefore the DM decides.
For most practical purposes, it doesn't matter
If a character is under the effects of 2 (or more) Bless spells, then they can choose to use one and that one no longer affects them. They can then use another on a subsequent roll (and so on).
Bless has a duration of 1 minute which is plenty to cover any single combat so deciding which Bless was used and which wasn't has little practical effect apart from knowing which caster has to keeep concentarting and which doesn't.