In the following example you’ll hear the female singers of the Morgana Chamber Choir training their awareness of each other in a choral improvisation exercise.
There are 2 leading singers improvising and interacting. The other singers start by echoing or doubling one of the leading voices, obliging them to listen in very closely to the voice they are following, and holding back their own voice. Gradually they may choose their own notes, as long as a listening connection is maintained in regard to the 2 leading singers.
In our rehearsals we regularly take time for exersices like this. Here’s the YouTube link: