Hi Huang,
Two ways:-
1. Create two different universe for each database and can use in webi by multi data providers . But there should be a common dimension.
Refer this thread:- Can create Webi report using multiple universe (multiple datasource)
2. Create one universe by using multisource connection (i.e for both databases) and then create webi on the top of that.
refer this link:- Information design tool 4.0: Concept - Create a multisource-enabled universe
Regards,
Nishant