no special trickery is needed. and triggers to keep the tables updated should definitely be avoided; total over-kill.
as rudy mentioned, you can reference tables in different databases by pre-pending the name of the database. just make sure that whatever user you use to log in to the mysql server has the necessary rights to both databases.
and i strongly suggest you use table aliases since you can't use the database.table.column notation with mysql. so do this:
select p.name as product_name
, m.name as manufacturer_name
from db1.products p
join db2.manufacturers m
on m.id = p.manufacturer_id