Respectfully, I advise clients to work with, not against, Asana so long as they’re using it.
It would be nice if Asana offered multiple assignments but they don’t and it appears unlikely they will, or certainly not imminently.
Subtasks have pros and cons:
but outlawing their use removes the best solution for both multiple assignment (including “Assign Duplicates” feature with subtasks; see other posts I’ve made on that if interested), and the workload (and other) workaround of multihoming the subtask into a (hidden) section in the parent project.
My workshop video in the linked post above may help with more complex workflows like you have, if you’re sticking with Asana, along with the other workaround posts related to the above.
Larry