How to dynamically assign parallel To Do task items in SharePoint Designer

I have a workflow that I'm creating in SPD which needs to allocate a sequence of parallel To Do tasks to a number of reviewers.

If all the reviewers were allocated To Do tasks all the time that would be fine.

However, based on the information stored in the list item the workflow is acting on, only some of the reviewers should be allocated tasks.

So Logically:

If Reviewer 1 selected, then allocate parallel to do task
If Reviewer 2 selected, then allocate parallel to do task
If Reviewer 3 selected, then allocate parallel to do task

etc.

Any combination of reviewers is possible.

I have found this impossible using SPD logic, and any help would be greatly appreciated.