Burnley's former Scotland Under-21 international MacDonald, 22, started a second loan stint with the Pilgrims at the end of August and will now stay at Home Park until the first week in December.
Aldershot striker Madjo, 28, will remain with the Pilgrims until mid-December. He has made seven appearances to date, four of which were starts.
Madjo's solitary strike came at the beginning of this month when his equaliser secured a 1-1 draw at Wycombe.
Pilgrims boss Carl Fletcher told the club's official website: "It's great that both Macca and Guy want to stay here so we have got it all signed, and they're now here for the next couple of months. They have settled in well and added something to the team and that's exactly what you want from loan players."