You should put some script in the Exiting event of your checkbox field.
Something like :
Dim ws as New NotesUIWorkspace
Dim uidoc as NotesUIDocument
Set uidoc = ws.CurrentDocument
if uidoc.FieldGetText( "YourFieldNameHere" ) = "YourValueToTest" then
Call uidoc.FieldSetText( "YouOtherCheckBoxNameHere" , "YourValueToSet)
Hope it helps…