Hello @anon82684441 and welcome to the Asana Community Forum
Having multiple assignees per task is not possible yet, but I recommend upvoting here
Now in terms of custom field yes definitely you can set it up via those and then once the form result flows into the project from where you can set up various rules that move tasks around, assign, multi-home, etc
Now let‘s say you want rules to run automatically for every new task that comes into this project, however you have other tasks created in this project also, then you have to play around with the rules to let them run based on other factors such as the custom fields.
Or else have a look into Flowsana that helps you customize things further if needed.