I've never encountered this. My inclination would be to ask the other reviewer if they're OK with the request. If they decline, then that would be final. After all there is usually nothing stopping a reviewer from identifying themselves if they wish; I certainly know of people who gave away their reviewer duties during private conversations.
It probably won't hurt to ask, because the worst that can happen is the editor says "no". If you do ask, I would definitely give the reason, since otherwise it's rather suspicious. You may also have to reveal your own identity first.