"It" what?
What two things? Same day. Same God. Same people. Same devil.
Correct, both established within the same time and era. Both part of the eternal and everlasting Covenant that was established by creation, law, and grace.
Shem proved to be under the new Covenant of Grace and the others remained condemned under the Covenant of Works (Law).
The Abrahamic covenant is the result and part of the new Covenant of Grace.
Right. As said, these remained condemned under the Covenant of Works.
There are only two Covenants. One of Law and the other of Gospel. These have existed simultaneously since the beginning.
Dispensationalists bring confusion into Covenant Theology, by insisting on numerous differences in eras of time, where it is claimed all kinds of doctrines change, come, and go. Not so.
From the beginning, all souls are condemned under the Law (Ephesians 2:1-3) unless and until God in His mercy and grace forgives them through faith under the new Covenant of Jesus Christ. (Ephesians 2:4-10)
This saving miracle has been performed and fulfilled by God since the days of Abel. Genesis 4:4 Many of the saints saved by the grace of God through faith, prior to the Incarnation, are listed in Hebrews Chapter 11.
Some things can be different while still the same . . .
The Law, the Gospel, and God's saving grace never changes.