Since bcf' s roll is locked how does that interact with the multiple attack depletion?

I know flurry dont activate the depletion but what if flurry is not your first attack?

If it's locked it can't be modified

A locked value can't be modified, the damage depletion modifier modifies the damage value, so it will have no effect on a locked value. BCF is unaffected by the DDM.

The first attack and second attack are both subject to the DDM, the first attack just won't trigger the DDM to reduce the second attack to a lower value than the first. So, if you attack once previously in the turn and then flurry with that character, the first attack of flurry will be at -1 and the second attack will be at -1 because the first attack of flurry doesn't trigger the DDM. The next attack that character makes will now be at -2, because we count the original attack and the second attack within the flurry (but not the first).