The way we have worked around this in the past is join 2 fields in one and use the other for a new parameter. For example we joined the "Drawn by" & "Checked by" fields to one "Drawn / Checked" and filled both names in a single field seperated by "/". This way we could use the other field to add the revision code we are using which contains both numbers and letters.
Hope this gives you an idea to workaround your issue.