David,
Great question. With Source No 1 defined as the default for new events, one option would be to deselect your first source from the Calendars tab in your side bar. With only, a new event created on the calendar will be created a new event in your second source.
For more on how event creation is handled with multiple sources, including info on adding custom buttons to create a new event in a particular source, see this doc:
http://www.seedcode.com/pmwiki/index.ph ... pleSourcesI hope this helps! Let me know if you have any other questions on this.
Dan Wheelon
[email protected]