I don't know if Fusions' power works that way. We know that a Fusion's power isn't just the sum of its two parts (
Vegetto was a LOT more powerful than just "Goku + Vegeta," and Goten plus Trunks wouldn't have been squat compared to Super Buu), so where does that extra power come from? Is it magically added by the fusion process, or is it something drawn from within the fusees' untapped potential?
We may never know exactly how fusion works (
this isn't Naruto; we don't get an in-depth, 3-page-long explanation about every single technique 
), but I think it's safe to assume that Fusions don't
need their "potential" unlocked, even if it hasn't been done already in some way.