Any advantages if we use multiple database instead of only one in PostgreSQL?
If it needs to be cross-verified, put it in the same db.  That way you can do your cross-verification in the db, taking advantage of indexes, query optimization, etc. which you can't do as easily in separate db's without a fair bit of extra work.

In theory you can spread out your disk/IO loads by putting it in another machine but typically, if this is virtually all write, I don't think it will be worth it.

If it is a different db on the same machine, you will not get any better write performance and you will likely get worse read performance.