What are the performance implications of storing some data across 2
databases instead of duplicating this data in both databases? ie. is
example 1 significantly slower than example 2:

Example 1:
SELECT Users.User.*, Transactions.Sales.date
FROM Users.User, Transactions.Sales
WHERE Users.User.id=Transactions.Sales.userid;

Example 2:
SELECT Users.User.*, Users.Sales.date
FROM Users.User, Users.Sales
WHERE Users.User.id=Users.Sales.userid;

Note in Example 2 the database Users would be significantly larger as
it has data duplicated from the database Transactions.

Thanks for any help.

If you can put the thing in one database that would be better.