Synchronous logical replication

Hello,

I have a question about synchronous logical replication.

There is the following description in the document "49.8. Synchronous Replication Support for Logical Decoding".
> A synchronous replica receiving changes via logical decoding will work in the scope of a single database.
> Since, in contrast to that, synchronous_standby_names currently is server wide,
> this means this technique will not work properly if more than one database is actively used.

What kind of problem will it occur?
For example,

publisher:
db1=# \dRp
                                   List of publications
    Name     |  Owner   | All tables | Inserts | Updates | Deletes | Truncates | Via root
 -------------+----------+------------+---------+---------+---------+-----------+----------
 pub1        | user     | f          | t       | t       | t       | t         | f
(1 row)

db2=# \dRp
                                   List of publications
    Name     |  Owner   | All tables | Inserts | Updates | Deletes | Truncates | Via root
-------------+----------+------------+---------+---------+---------+-----------+----------
 pub2        | user     | f          | t       | t       | t       | t         | f
(1 row)

synchronous_standby_names = 'FIRST 2 (sub1, sub2)'

subscriber:
db1=# \dRs
             List of subscriptions
    Name     | Owner  | Enabled |  Publication
-------------+--------+---------+---------------
 sub1        | user   | t       | {pub1}
 (1 row)

db2=# \dRs
             List of subscriptions
    Name     | Owner  | Enabled |  Publication
-------------+--------+---------+---------------
 sub2        | user   | t       | {pub2}
 (1 row)

In the above example, will not synchronous replication for both sub1 and sub2 work?
Also, if sub2 is changed asynchronously, will any problem occur?

Regards,

--
Takanori Asaba