Is there a way to set the approving order for serial assignees (number undefined)? For example, assign a task to multiple assignees User1, User2, User2, and let User1 works on that first. Then, User1 hand over to User2 once finishing. User2 do the something same to User3 …
Yes, but it will take some work. I believe multi-value fields are guaranteed to return in the same order they are set. If so, you could keep track of your reviewers in a multi-value field. Then, in your business process definition, you set up a little loop that pops a reviewer off the top of the stack, queues a task to them, keeps track of their response (approve, reject, etc.) and then moves on to the next one in the list until the list is empty.
To let your user actually set the value of the field, you can look at the OOTB controls for working with multi-value fields. If that is usable enough, there you go, otherwise, you'll have to create your own little form control that works the way you want it to.