In this particular case -
these are 2 different John McDonald's who both left the Glenurquhart area and both settled in Pictou County NS.
The John (Iain MacEoghainn Oig) I am descended from left 1st (in 1775) and the other John left in 1801 (according to the Bryan Keddy Glens website).
However, I believe both families are now connected through the marriage of Iain MacEoghainn Oig's daughter Christy, who married the son (or brother) of John 'Og' McDonald, Duncan 'Speich' (Speech or Speid?) MaDonald