So what exactly is the question? If you should use two? You don't need two.
The one on the rail is just a matter of convenience. Use it to look at while you adjust pressure. Other than that, it serves no purpose. Usually these little ones are not all that accurate and maybe even a source of a potentially dangerous leak.
The other one should be accurate and where you can see it when driving to make sure the fuel pressure does what its supposed to do.