No. Sifter is designed with a heavy focus on accountability. In our experience, having multiple assignees dilutes that accountability. By requiring that each issue only have one assignee at a time, there's never any question who is responsible. If that assignee is currently being blocked by a dependency on someone else, they can simply reassign it to that person, and the ball's in that person's court. We've found this keeps things moving forward as each issue becomes a virtual hot potato of sorts.
However, Sifter does offer a potential solution to help mitigate the need for multiple assignees. For each project, you can configure a handful of team members to receive "unassigned issue notifications". Whenever an issue is created but not given an assignee, everyone on this list will be notified. This can be particularly useful if your team leads or project managers review issues before prioritizing and assigning them to other team members.