When Voldemort is about to break into Dumbledore's tomb to take the Elder Wand, the following happens:
"I shall join you in the castle shortly," he said in his high, cold voice. "Leave me now."
Snape bowed and set off back up the path, his black cloak billowing behind him. Harry walked slowly, waiting for Snape's figure to disappear. It would not do for Snape, or indeed anyone else, to see where he was going.
This seems to indicate that Voldemort did not want Snape to know that he was taking the Elder Wand from Dumbledore.
However, a month or so later, during the battle at Hogwarts we find the following dialogue between Voldemort and Snape:
"Why doesn't it work for me, Severus?"
In the silence Harry imagined he could hear the snake hissing slightly as it coiled and uncoiled – or was it Voldemort's sibilant sigh lingering on the air?
"My – my lord?" said Snape blankly. "I do not understand. You – you have performed extraordinary magic with that wand."
"No," said Voldemort. "I have performed my usual magic. I am extraordinary, but this wand... no. It has not revealed the wonders it has promised. I feel no difference between this wand and the one I procured from Ollivander all those years ago."
Here Voldemort jumps right into the discussion about why the Elder Wand doesn't work for him, without first explaining to Snape that he had procured the Elder Wand. This implies that Voldemort expected Snape to know about it, which would indicate that Voldemort had in fact shared this secret with him.
So what changed? Why/when did Voldemort go from hiding it from Snape to sharing it with him? It would be one thing if he only told Snape about it then when he was planning on killing him (so he wouldn't care if Snape knows), but he seems to have changed his mind and told him before this occasion.