Unfortunately, it's not possible to do that. You can only make one person responsible for one task, but this person can of course create sub-tasks and assign each sub-task to someone else.
We may consider changing that and you're welcome to vote for this request in our Votebox: https://todoist.com/Vote/show/7603/
Is it now possible ?
It's still not possible to make more than one person responsible for the same task. We'll consider changing this in the future.
Please do this