Sacrificing a creature is a special action or cost that moves it from the battlefield to its owner's graveyard. Once a permanent is sacrificed, it ceases to exist on the battlefield (CR 701.17a). There is no 'simultaneous sacrifice' mechanic — each sacrifice is a discrete event that consumes the permanent.
If two different effects or activated abilities each require you to sacrifice a creature as a cost (for example, Phyrexian Altar and Ashnod's Altar), you must pay each cost separately. You can sacrifice one creature to one altar, but that creature is already in the graveyard and is no longer a legal target or cost-payer for the second altar (CR 117.3, 117.6).
Costs must each be paid with distinct, available resources. You cannot use the same game object to pay for two separate costs, because after paying the first cost the object no longer exists on the battlefield (CR 107.1 analogy; CR 117.3b on paying costs).
Example: You control Ashnod's Altar and Phyrexian Altar and only one creature, a Goblin token. You can activate one altar's ability, sacrificing the Goblin token to pay that cost. The token immediately goes to the graveyard. You cannot then activate the second altar's ability using the same token, because it is no longer on the battlefield.
If you want to benefit from both altars, you need two separate creatures to sacrifice — one for each altar.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.