I would suggest you start them at the same time. You don’t have to, of course, but there wouldn’t be any harm in it.
The Genesis handbook spends two days reading the Creation account in Genesis 1 and 2 and asks for an oral narration. After those two lessons, you move on to the rest of the accounts in Genesis through Deuteronomy.
In 106 Days the child draws an illustration of each day of Creation in lesson 1 then you start in studying more scientific aspects of what God created. So there wouldn’t be much overlap at all.
Does that help?