Following their divorce, my father paid his ex-wife a compensatory allowance, in the form of an annuity. He has passed away. I learned that she was going to receive a survivor's pension. Are these two services cumulative?

No. When the compensatory allowance was paid in the form of an annuity, at the time of the death of the debtor spouse, in principle it is immediately payable in the form of a lump sum. Reversion pensions are deducted from the amount of this compensatory allowance (art. 280-1 and 280-2 C. civ.).