Sacrificing dilemma

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.

This is correct.
In magic you cannot sacrifice 1 object to several effects.

In this case they are both payments (costs) for an ability.

Once you activate one of them, until you pay the cost (this is done after you annouce the ability, put on stack, select the modes (if any), select the targets(if any), determine costs, get mana from sources, pay costs), you cannot activate another. At this moment one of the creatures you sacrificed for the ability is no longer on the battlefiled so it cannot be sacrificed to another ability.

Also whenever a spell requires you to sacrifice a creature, and you sacrificea a creature for an effect, you will still be required to sacrifice a creature for the original effect
Eg: Geth's Verdict says opponent sacrifices a creature. If he has that death cultist and another creature, if he sacrifices cultist to its own effect, he is still required to sacrifice a creature.

Hope this clears things out.