I tried to find a solution for this by first checking if I could have two Action element for same class and assembly but with different sentences like "find a document with id equal to %1 in list with guid equal to %2" and "find a document with path equal to %1 in list with guid equal to %2"... BUT this does not work because for a given class and assembly, sharepoint finds first appearance of ACTION element in the .actions file and displays that sentence only. so you may NOT want to take this route.
If you are willing to change the logic of your activity, here are some options.
sentence = find a document with %1 equal to %2 in list with guid equal to %3
%1 = FieldName (string) and
%2 = FieldValue (string) and
%3 = list GUID
(use your own names).
FieldName appears as a dropdown with value like "Document Path" and "Document ID" and expect user to select appropriate values. FieldName look like
<FieldBind Field="FieldName" Text="Input Type" DesignerType="Dropdown" Id="1">
<Option Name="Document Path" Value="Path"/>
<Option Name="Document ID" Value="ID"/>
Option 2 (I would prefer this)
sentence = find %1 document in list with guid equal to %2
%1 = FieldValue (string - so that path or id both can be provided)
In your code you will have to infer whether user provided ID or Document Path (I would try to parse as Integer and if success assume a id otherwise a path). On designer you can provide a hint to user using following:
<FieldBind Field="FielValue" Text="Document ID or Document Path" DesignerType="TextBox" Id="1"/>
Write two separate activities FindDocumentById and FindDocumentByPath (choose your own name, of course)