I may be wrong as I'm not extremely well versed in the rules, but I believe it is as follows:
For both of these cards, Sacrificing a Creature is a payment, you do not sacrifice at the time the spell resolves, payments for abilities and spells do not use the stack, they happen instantaneously when you need to pay for the spell or abilities, before they go on the stack, and then later, resolve. As such, if you sacrifice the death cultist to the dross hopper (at which point the gain flying ability goes on the stack,) there is no time to sacrifice the death cultist for any other purpose.
Now if the death cultist said "if death cultist is sacrificed, you may..." it would be a triggered ability, and you would be able to use that ability.