Legality of a single ADF approach depends on the category of airplane in the US. What kind and age of airplanes are you flying?
The US Air Force bought a bunch of 737-200 in the 1970s as training aircraft, and called them T-43A. They bought them with only a single analog-tuned ADF, as they were never expected to make serious ADF approaches.
As these things go, of course, the planes were pressed into VIP service. The USAF always had millions for new, glitzy projects, but pennies for valuable upgrades, so the T-43A never got the second ADF, let alone GPS, which was widely available to them by 1994.
That's when a T-43A based at Ramstein flew to Bosnia one stormy night with the US Secretary of Commerce and other VIPs. Their ADF approach required crossing one NDB, then tuning to another near the field. The pilots failed to correct for crosswind on the outbound leg and impacted terrain...
The analog-tuned ADF receiver was superseded by a digitally tuned analog receiver in the mid-1970s. The analog-tuned unit at least had a nervous needle, which signaled the pilot it was working, unlike the later unit with its well-damped, seemingly dead needle.
GB