This answer here makes reference to there being no obligation for NATO members to defend eachother under article 5 if the agressors are both members of the treaty.
Having read the article in question, an attack on one shall be considered an attack on the whole, seems to be the general thrust. In this case, surely the article would require that the members defend the attacked country against the aggressor regardless of their treaty membership? Presumably, also kicking the agressor out of NATO whilst doing so.
So my question is thus, is there any part of the treaty which explicitly details what process is to be followed should two member states enter a state of war between eachother. It might also be interesting to consider how this might be different in situations where there is clearly an aggressor rather than a situation where the waters are far more muddy (e.g. the accidental downing of an aircraft that sparks an overzealous response).