Interesting analysis, as usual, Ed. Consider this expansion. She started combing in the bedroom; while she was finishing up the combing, she walked towards the bathroom.The two 'ing' words do sound awkward together, but I wouldn't call the sentence wrong.
Omitting either of them would fail to convey the full meaning, which is that she had started combing her hair somewhere, presumably in her bedroom, but had not yet finished before she started walking to the bathroom. She was still finishing off the combing operation as she walked.
Even though it still contains both ing-words, this looks and sounds much more attractive than the original.while she was finishing up the combing