I can only think Lizzy and Mr Wickham were able to continue a 'whispering conversation' with Mr Collins between them because Mr Collins himself was talking across the table to a guest, possibly enrapturing on his noble patroness. Virtually all Mr Collins conversations appear to revolve round Lady C. To compare a room in a house with one of her smaller rooms, enthuse on the chimney piece, even take her advice to marry. So long as he is either conversing about Lady C. or musing on a vase which reminds him of one at Rosings he'd be sufficiently occupied.