It's not a "two-part belted plaid." After the philabeg replaced the belted plaid as the common garment - mainly after the Proscription was lifted - Highlanders working or travelling in the countryside still utilized "plaids" as upper-body wraps and blankets (so did Lowlanders,actually). These were of a smaller dimension than the old full-sized plaid, and carried in the manner of an over-the-shoulder blanket roll. You can see this in this engraving circa 1770:



This is what you see being done in the "Rob Roy" film, perhaps too early historically. However, the costume advisor for the movie was Peter MacDonald - a very knowlegeable fellow regarding the history of tartan and Highland garb....