The Work field is not customizable. Furthermore, fields that are customizable with formulas such as Number1-30, Text1-30, etc. can only use values from the task itself, not other tasks.
If there are only a few such tasks that need to be updated, do it manually. However, if there are many you'll need a macro to update one task based on another task. There are likely many ways to do this, what springs to mind is to tag these pairs of tasks with a code in a text field (such as Text1). The macro would loop through the tasks looking for a code that meets a pattern (e.g "Pair01_FixedWork") and then would search for its mate "Pair01_FixedDur" and update accordingly.
Note: the macro would need to be run whenever the driving task information changes.