the remaining 1.9 million (were) either owners or managers

You can, and it would be better.

The first, preceding "were" implies one in the place that you suggest, which was omitted for reasons of brevity and elegance, and I consider that it would not necessarily be "better".

There are six boys in my family. One is fairhaired, two are dark, and six (are) redheads.

The second "are" above may be omitted but implied and understood in the above example.

Beginning learners should avoid such tricks however, until they feel confident about the basics of English.

There are nine boys in my family. One is fair haired, two are dark, and six are redheads.

I got the numbers wrong! Also of course I meant the third "are", the one which is underlined. This can be omitted.