I Have Pedometer++ but find it is always different then what is listed in the health app. Often is off by a few hundred steps a day.
You should read David Smith's blog where he explains how Pedometer++ actually does a better job than the health app.You can look at the actual data in the health app. Apple will combine iPhone and Apple Watch data, attempting to ignore steps with the same time stamps, but counting both devices when they differ.
I have not tested Pedometer++ to compare how it handles combining (or not) steps from multiple devices, but I don't think it is currently possible for developers to use the same routine that Apple uses in the Health app. (At least it was not a while ago when I investigated this.)