Can pork and beef be thawed in water together?
This can be safe; mixing isn't fundamentally dangerous. The things to keep in mind are:


The overall thawing method needs to be safe. Ideally that means running water, but the key is that it's fast enough to avoid spending too long in the danger zone. A bowl of water is likely too slow, though it can be fast enough if the water is agitated and changed periodically to simulate running water. See also What are the acceptable methods to thaw food items?
They'll need to be cooked to the higher required temperature of the two. For example, if the beef were ground, it needs to go to 160F, so even though pork chops only need 145F, you'd probably want to cook them to 160F anyway.


The mixing doesn't really change anything, any more than adding raw beef and pork to the same pan does. You just end up with a combination of the unsafe things from both, and then you cook them both to render them safe. Safety is essentially just about cooking to a high enough temperature, and not staying in the danger zone (40-140F) too long. 

I don't think this is a particularly good thawing method, though. Ideally anything you've frozen is in an airtight, watertight package of some sort, so you could just keep both packaged during thawing. Or if you plan ahead, you can thaw in the fridge.