I have a similar problem. I also have a combo box that has only three values, Branch 01, Branch02, or BothBranches.
WHERE ((dbo_SalesHisItemPcVw.Branch)=IIf([Forms]![frmRollingDates]![cboBranch]="01","01",IIf([Forms]![frmRollingDates]![cboBranch]="05","05","01" Or "05"))
If the user wants both...