Do multiple axons innervate a single skeletal muscle fiber?
Until a better educated person contributes a more detailed answer, Hunt's article here from 1954 will likely be the best source,


  studies have shown that muscle fibres in the frog's sartorius are innervated in two major zones and most of the fibres receive motor
  endings at at least two regions along their length (Katz &amp; Kuffler,
  1941). In this case a multiply-innervated muscle fibre may derive its
  several end-plates from the same motor nerve fibre. The possibility
  also exists that a muscle fibre may receive end-plates from more than
  one nerve fibre, i.e. polyneuronal innervation.


HUNT CC, KUFFLER SW. Motor innervation of skeletal muscle: multiple innervation of individual muscle fibres and motor unit function. J Physiol. 1954;126(2):293–303. doi:10.1113/jphysiol.1954.sp005210

He in turn references an article by Kats and Kuffler from 1941,

KATZ, B. &amp; KUFFLER, S. W. (1941). Multiple motor innervation of the frog's sartorius muscle. J. Neurophysiol. 4, 209-223.

As those are both 70 years old, it is likely that more documentation has been produced, and that it could contribute to a competent answer to the question.