‘ And Joshua spoke to the house of Joseph, even to Ephraim and to Manasseh, saying, “You are a great people, and have great power, you shall not have one lot only, but the hill country will be yours, for though it is a forest you will cut it down and its goings out will be yours, for you will drive out the Canaanites although they have iron chariots, and although they are strong.'
Joshua then made a prophetic declaration. He first gave them a feeling of their great power and importance. Let them cease their fear and as a large people look to YHWH Whose power they could enjoy. Then he forecast that they would indeed cut down a forest to make way for themselves among the mountains and would eventually defeat the Canaanites with their iron chariots. Certainly later Megiddo and Taanach did become available to their control.
“You shall not have one lot only.” This did not meant that they would receive a second lot but that they could make for themselves a second lot by utilising what had been seen as unusable land.
“Its goings out shall be yours.” They would not be confined to the hill country but through victories over the Canaanites would be able to go out into the plains. They were thus to have faith in YHWH and go forward.
“Even to Ephraim and to Manasseh.” Note the change of order from Joshua 16:4. What followed Joshua 16:4 had in fact put Ephraim first. As with the blessing of Jacob the position of the sons was transposed. This would later be the established order.
So all these words were included in the narrative as an encouragement for God's people to take the initiative and make opportunities were they do not seem to exist, and not to sit around moaning but to go forward in faith. Then all will open up to them, even what seems unachievable.
It is a reminder to us that lack of opportunity often arises from our unwillingness to look around and see what is available. We want to have it easily, without effort. Thus we miss the opportunities that are there.