Joshua 4:23 KJV
For the LORD your God dried up the waters of Jordan from before you, until ye were passed over, as the LORD your God did to the Red sea, which he dried up from before us, until we were gone over:
The same God that brought Israel out of slavery will bring them forward into newness of life.
There are at least two ways to view the two partings:
First, as a nation.
It was through this impossible feat that God delivered them and through the same impossible feat that God gave them the land of the Canaanites.
The God who saves us is not shortened after our salvation. After forty years of living after the flesh Israel needed to see, as we do today, that God is very able to do exceeding abundantly above all that we ask or think.
The second view is to take into consideration that we are looking at a brand new generation.
Those who had crossed the Red Sea were all dead by now. These are their children and children's children. 1 Corinthians 10 says that the Red Sea was a baptism. Now it is their children's turn. They cannot go on their parent's baptism. What is the spiritual experience of Mom and Dad does not translate to their children. Each generation must meet God personally and each new generation must follow Him as their own God.