How can I create a Full Text Search catalog on 3 fields where one is in a second table?
You'd have to create a view and create an FT index on this view
From MSDN "Getting Started with Full-Text Search" (my bold)
I haven't tried this myself so don't have any tips. However, an indexed view is the only way to have any index span tables.
The only way to do this is to create an "indexed view" that combines the two tables into a view. You can then build the full text index on that.
Here is the documentation you need direct from Microsoft.