Wit1,
While I've not tested this scenario, Rebuild deletes the FT Catalog &
re-creates it, but does not run a Full Population as start_full does
executes a Full Population. Start_full removes all entries, but does not
delete the FT Catalog, the difference between rebuild vs. start_full is akin
to DELETE vs. TRUNCATE for a SQL Server table. Basically, there is no need
to run a Rebuild prior to a start_full (unless you need to physically
deleted the FT Catalog files for some reason), and the best recommendation
is to only run the following:

use Database
go
EXEC sp_fulltext_catalog 'catalog_1', 'start_full'
EXEC sp_fulltext_catalog 'catalog_2', 'start_full'

Hope that helps,
John