I'm affraid you will not be able to do it in an easy way with multi selection list.
I would suggest to use repeating section, where at every repeating section row you will select single department and make lookup for approver(s) for that single department. with repeating section you can define as many department/approver pairs as you need.
then place calculated control value outside of repeating section with formula referencing approver control within repeating section and it will collect you all of the approvers.
it might look like this
Hi Marian
Thanks for the suggestion. I seem to be making some progress following a setup you described but am just stuck on converting the information to a String that can be used to assign a Flexi Task to.
So I've done the following.
1. Put a calculated value in each repeating section that will retrieve the correct Approver for the department. It is retrieving it from a Colum Type of User. Calculated value can only store it as String.
2. Created a form Variable String that holds all of the Approvers.
3. In the workflow, QueryXML is retrieving the form variable.
The value returned however, is in the format of a2;#John Doe,30;#Peter Pan]
So I'm getting my list but somehow need to convert that to type User or some other valid output.
Any ideas?
yeah, that's how lookup function works.
I think easier will be if you convert user identifiers already in forms.
so you will have to write some javascript function that will do that.
some ideas how to do that you can find eg here javascript - Get SharePoint user id from login name - SharePoint Stack Exchange
Kinds of defeats the purpose of the entire product when the "easiest" solution is to just write up some JavaScript.
Finding a few glaring flaws in what is an extremely powerful product, yet can't do some easy and basic tasks. We are leveraging Nintex because we don't have the local developer resources or the programming skills to code complex workflows.
welcome in nintex world, some thing are sometimes really hard to understand why they designed the way they are or why there are missing very trivial functionalities/features 
if you have problems to customize forms with javascript then I would have another suggestion/approach for you:
- let the user select just departments in the form
- perform lookups for approvers within the workflow. if you need that you can then update list field with that list of approvers, if not you can use it just within workflow to assign them taks(s)
Hi Frank
Unfortunately this question can be closed as self resolved.
I ended up doing some fiddly lookups that joined values to a collection then looped through and converted these values to user type variables and rejoined them back to a list of approvers. Not the cleanest and easiest way of performing multiple lookups on a form but to this day I have not yielded an easier way of achieving this.
