Indexterms with one or more See elements

While working on a legacy project I found the following to be valid:

<indexterm><primary>Engineers</primary><see>Institution</see></indexterm>

<indexterm><primary>Engineers</primary><see>North of England</see></indexterm>

This produces an entry that appears as

Engineers (see Institution) (see North of England)

and usually the see would italicized in an index. 

However, the following is does not validate.

<indexterm><primary>Engineers</primary><see>Institution</see><see>North of England</see></indexterm>

On the page, http://docbook.org/tdg51/en/html/indexterm.singular.html, there is reference to "if there is at least one see entry for a particular term." This to my mind implies that there may one or more see references.

When consulting the Chicago Manual, Edition 17, the first example is an entry with see two different entries, as follows:

adolescence. See teenagers; youth

Edition 16, however, has only single see entries in its' examples.