>>

select Table1.fielda from Table1 left join db2.dbo.Table2 on Table1.pk = Table2.pk where table2.field2 = '1234' 
-- assumming the schema is dbo and the database is db2 for table 2

>>
>>if is not in the same instance then you will need to create a linked server on the first instance and add the linked server in a four part naming [linked server].[database].[schema].[table]
>
>I think when you are saying "instance" you mean instance of SQL Server. In my case, it is the same instance of the SQL Server but different databases. As in your code, 'db2' is a different database on the same SQL Server.
>And I suppose that since SQLEXEC uses the handle for one database, the code won't work. Am I understanding correctly?

It will work. The dbname.dbo.table.field allows you to use tables from other databases on the same instance. For that matter, you don't really need two handles if you use that syntax.