Your last node/control point of the active path is selected, go over the last node/control point of the over path you want to joint, Hit the Ctrl key you will see a double "ring" looking like the "infinite character" ∞ , then just click, the two path will be joined
if your path are on a different "layer" in the path tab, you will need to merge them (right click > merge visible paths)
if your path are on a different "layer" in the path tab, you will need to merge them (right click > merge visible paths)