I had second thoughts about doing it this way and am instead trying to implement a repeating table for the list. The table has two columns, one add, one remove. It works fine (concatenating from the repeating table into text boxes). I have the mutually exclusive bit working, but now I have the problem of not having the boxes checked if the form is opened in Edit mode. I have a model working that isn't tied to a SharePoint list (based on a sample from this very forum), but can't seem to apply the same solution to the list.
What I'm looking for is something like this:
Options Add Remove
Charger X
Phone case X
Car Charger X
The options list can be any length (it's fed from another SharePoint list). Users should be able to choose to add or remove (but not both) any item on the list (I have this part working). It's also possible for a user to change the request before it's complete. In this case, the form comes up in edit mode and all the same rules apply, but the previously selected changes should be reflected on the form.
Does anyone have thoughts as to how I might accomplish this without code?