We haven't yet, but I suggest we don't find out. Given what each of them could individually, them resonating with each other could spell disaster.

I don't think the undead is who we need to be worried about, though. I know Master Kaneki was supposed to announced it to everyone, but it seems he may not have had time before he disappeared.

This is the White Traveler's heart. The one who is currently holding the child, and who seems to be the one who 'made' all these undead Lords we keep meeting. That's why we should keep it safe, and why our merchant friend calls it 'leverage'.