alternative branch in collaboration diagram

[Royski]

I'm a newbie but I'm led to understand that conditional paths can be drawn in collaboration diagrams. i.e. 2a[modemResponded]:setBaudRate with an alternative path 2b[timeout]:sendErrorMessage. Problem is that if you add a message to an association on the diagram the numbering order is added automatically, leaving you powerless to add 2a followed by 2b. Instead, you're left with 2 followed by 3. I can't see anyway of overriding the system to add your own numbering system.