Since table A and Table B are linked in a one to one relationship. The simplest solution is to merge the two into a single table.
Other than that, you'll need to use one of the following options:
Always create a new record in B every time you create a new record in A. (A script can manage this option for you.)
Remove the field from B and define it in A
Define a calculation field in A that refers to the Value of Done and sort on it.
That table is already way too wide with fields and I don't want to contribute to the problem.
Sometimes you have no reasonable alternative but to add more fields to your table. Hundreds of fields in one table is not out of the question in some situations.
We have talked about implementing a script on record creation, just haven't ever gotten around to it. That would probably be the best answer, but also the most time consuming to implement.
Why would it be time consuming? Setting up a new record button or a custom menu ( if you have FileMaker Advanced) with your script in place of the standard new record option is pretty straight forward to do.