Stephen, in Acts 7, is taking the religious leaders of the council, before whom he is on trial, through the history of Israel to prove that he is not a blasphemer but is simply following the trajectory of their faith. That his belief is in the same God and his plans and promises made to Abraham and the Patriarchs and fulfilled in Jesus Christ.
In Genesis 12 you see the great Quad Promise: People, Place, Protection and Plan. In Genesis 15 we see it reiterated and expanded. It is not that these are the two times that God appears to Abram, rather as we saw in chapter 12 Abram takes this journey with God, he enjoys an ongoing relationship with God. Chapter 15 zeroes in initially on the people promise, the promise of a son(1-6), and ends with this verse "And he believed the LORD, and he counted it to him as righteous." Abram wasn't righteous by default, he was righteous by grace. He wasn't righteous by his actions he was righteous by faith in the promise God made. Abram's faith is just like Stephen's, just like ours, not earned but credited.
In verses 7 and 8 the focus shifts on to the promise of land. How can Abram be certain that his descendants will inherit the land? God does something amazing, certainly it would have astounded the Israelites as they read of Yahweh's actions here in the years post Exodus. Yahweh is a God unlike any other. Yahweh makes a covenant with Abram, but it isn't a covenant that bilateral. God alone is the one who passes through the corridor of carcases all so that Abram can "Know for certain that your offspring will be sojourners in a land that is not theirs and will be servants there... And they welcome back here in the fourth generation..." God provides staggering detail of events in the future, some four generations away, showing Abram how he will act in history. And (17-21) as the sun sets God seals the covenant as the smoking firepot and flaming torch pass between the offerings. God solemnly swears to do what he has promised. And God keeps his word, he is faithful. And faith banks God's promise certain that he will do what he has said.
God holds history in his hands, he is sovereign and rules and reigns, he can be trusted to keep his word. To Israel post exodus as they journey to that promised land what an encouragement. To Stephen it is an encouragement that God kept his promises to Abraham, and to the people, and that has led him to faith in Jesus the end point of those promises. Faith banks God's promises and lives in the light of them. As God's people we similarly can trust God and the promises he has made to us because they sealed once for all not in his passing through a corridor of carcases but in the far greater covenant sealed with the blood of his Son.