Nina Conti is talented, in the art of ventriloquism.
I am certain that the two audience's persons were already chosen for their 'undisclosed' rehearsal.
I understand what you're saying, but for me personally it takes a little bit off from the overall performance.
It's just that I am aware and it distracts from the spontaneity, the honest and natural fluid of events, the true 'improvisation'.
This, is my own personal opinion. I still enjoyed it, and laughed too; less and less though as my awareness became evident, so, not to the full extent.