The VOR check is required by 14 CFR 91.171. The dual VOR check may be done in place of other ground or airborne checks. No guidance is provided detailing weather this check may be performed in the air or on the ground. Lack of this guidance means the regulation must be interpreted as a plain english rendering, or in other words, literally. Lacking guidance for the use of the dual VOR check with regard to airborne or ground, it may be done either way.
FAA Publication P-8740-18 provides no additional guidance.
AIM 1-1-4 provides no additional guidance regarding a dual VOR check, but does note that yearly calibration should be performed on the equipment, becasue deteriorating equipment condition can cause an accurate reading close to a VOR, but may be inaccurate farther from the VOR with a weaker signal.
You should be 20 nm or more from the signal for a good test. For a 91.171(c) ground check, no specific guidance has been provided with respect to being on the ground or in flight, or distance from the station. Accordingly, there is no such requirement.
The FSDO level has no authority to either interpret regulation or add to it. Finding inspectors at the FSDO level who do not understand the regulation is a common occurence.
What you do need to ask yourself is w(h)eather or not this fight is worth disagreeing about, or if you should merely do as the POI asks and wait for something important to fly your flag. Your choice.
If you're deadset on going toe to toe with your POI, request clarification on his statement; have him show you why a ground check while complying with 91.171(c) is illegal.
