A double check valve is merely two single check valves in close series. This means that under negative pressure the water that momentarily bypasses the first check valve and causes it to close is stopped from going any further by the second stop valve which will close at exactly the same time (assuming no air or dirt in the valve and pipe). A single check valve stops backflow, but miniscule amounts of "contaminated" water will have passed back along the pipe before it closes. In theory two single check valves in series anywhere along a pressure pipe will act jointly as a double check valve, but the farther they are apart the more likely it is that something will act to stop them doing so. Possibly the shower maker has found it easier to adapt a design by using single check valves either side of the mixer instead of a DVC elsewhere, perhaps on the grounds of length of a DCV fitting. Because of the typical design of both types of check valve they can restrict forward flow rates noticeably, and you should only have them if you really need them.

Now, the reason that double check valves are is there is purely to stop pollution of the mains - not your domestic cold water supply, and as such it is a complete con - the water network is raddled and full of cracks leaks and miscellaneous holes, from which about 20% of the water put into supply escapes. Therefore negative mains pressure (eg caused by a burst pipe downhill of you, or the fire brigade pumping water faster then the main can supply) will permit dirty groundwater to infitrate the mains pipe - possibly including road run-off, contaminated groundwater from industrial activities, or sewer leakage. The idea that public health is threatened by your shower dangling in the bath is cobblers, and as such you shouldn't be overly concerned about the arrangements.