Diamond doves.... 1 hen or 2?

[Zebrahead 3]

I have 1 cock diamond dove, his partner died a few weeks ago, and he has teamed up with my pair of Java Sparrows and all get along very well ( i think he thinks he's a Java ) but recently he has been cooing quite a lot and I don't like to see my birds without a partner, I always buy as pairs. 

Do I get a single hen bird or maybe 2 hens, would this encourage breeding or cause problems?

also thinking of another pair of Java Sparrows, I had a single pair of canaries and they wasn't interested in each other, but when I got a 2nd pair the singing increased and now nest building is on the go, so will the same happen with the javas ?