if you have trouble getting the feeling of your double stops using this method, it's a good idea to check your open strings with a chromatic tuner. If you are tuning your 5ths too wide, the G string will be flat, so if you use the resonance of the G or D to tune a double stop on the upper strings... it will sound out of tune.
This gets into the whole issue of temperment, which is a very long and complicated topic, but the important thing to remember is this: "if your strings are in tune, and one note of the double stop rings with the open string, then you can trust it, and adjust the other note until you have a clear sounding interval"