This has come up here recently...you might want to use th search in the blue bar above. Basically, most valves (but not all) meet today's code requirments. This requires it to be a pressure balanced valve to prevent a water pressure or supply problem from scalding somebody in the shower if, for example, someone flushes a toilet. It is easier to do this in a single handle fixture. There are a few two control fixtures, but they generally cost more. Depending on where you live, if just repairing an existing valve, you don't have to get a permit or update the system to today's code (although it is wise to do it). But, some places require you to do this. You can buy replacement valves that do not meet today's code, but they are getting harder to find. Depending on how far apart the handles are, some companies make remodel plates that are big enough to cover many old handles.