two separate NDBs simultaneously (in line)
Only possible in the event that there are two separate MF beacons in line with the approach; not always the case.
When I started flying I predicted that by the time I retired both NDB and HF would, like the three pointer altimeter, be obsolete; regrettably I was wrong.
On some older installations pilots at least had a large prominent radio compass or RMI and were in regular NDB approach practice. Unfortunately, later installations buried a tiny RMI with no QDM bug in a corner of the instrument panel where it is more difficult to scan and, in any case, the pilots rarely get the practice of flying a genuine NDB approach.
Used to operate into a grass strip where the guys built their own MF transmitter and used crosscuts on a nearby VOR for descent - worked beautifully. Needless to say I never used this unapproved facility.
In answer to the question, I recollect that the answer is yes because we'd sometimes have one RMI tuned to the approach NDB and the other tuned to the g/a NDB. I don't have a MEL to hand to see if the majors for whom I flew permitted an NDB approach with one receiver u/s.