Mentioning other people's error in talk
First, make double sure that the error is on their part, not your part, or there is not another reason for the discrepancy (e.g., they're computing something slightly different, they're using different assumptions, there's a simple way to transform their answer into yours, etc).
It seems reasonable sometime between now and then, to contact the authors (1 and 2), to check in and say you want to mention their work in your talk but you're concerned about this possible error.
Try to talk to Author 2 at the conference before your talk about this issue.
What to actually say depends on the situation and exactly how related their work is to yours, but if you are not getting any responses from them, you might just mention their work in passing.  You could avoid saying anything definitive about their work.  You can also just say that there seem to be discrepancies in some cases and you're looking into it.  I would avoid directly saying their work has errors unless they specifically admit that it does (in which case you still should not harp on it).