Can a query only use one index per table?

I have a query like this:

( SELECT * FROM mytable WHERE author_id = ? AND seen IS NULL )
UNION
( SELECT * FROM mytable WHERE author_id = ? AND date_time > ? )

Also I have these two indexes:

(author_id, seen)
(author_id, date_time)

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I read somewhere:

A query can generally only use one index per table when process the WHERE clause

As you see in my query, there is two separated WHERE clause. So I want to know, "only one index per table" means my query can use just one of those two indexes or it can use one of those indexes for each subquery and both indexes are useful?

In other word, is this sentence true?

"always one of those index will be used, and the other one is useless"