Petar,
#1.First, create a 2-column database table with the following entries (following your example above):A/B,A/C,A/D, A/E, B/A, B/C, B/D, B/E, C/A, C/B, C/D, C/E, D/A,D/B,D/C, D/E, E/A,E/B,E/C,E/D (in short, it has all possible pairs except of A/A, B/B, C/C etc)
Such table can be easily created programmatially from your user list, and if you are good with SQL, you can even do it as a view to your dwuser table (using only column name). For Docuware Cloud system this would have to be done as a table on a local database accessable via local database connector)
#2 Create a 2-column Docuware Select list based on the table/view from #1
#3 Then, add an additional field to your DW FC and to your store dialog, and call it, let's say CurrentUser. In the store dialog make this field a prdefined entry = CURRENTUSERLONGNAME. If wanted, this field may be made 'hidden' in the store dialog.
#4. And, finally, make your payload field in your store dialog to be a select list only from the 2nd column of the #2 above while filtered on CurrentUser field from #3 being equal column1 from #2 select list.