With their SOULS combined, the two of them return to the surface together (and run into some old friends along the way) and reunite with their friends and family.
This article is based on the comic A Dreemurr Reborn by dreemurr-reborn. Go check it out.
These comics and pictures cover Asriel and Frisk’s journey together from where they join together, to their trip back through the underground.
After Frisk gave their soul to Asriel, Asriel woke up in the bed of golden flowers, seeing himself wearing the robe, in confusion. He sees Frisk, running to them, they remember that Frisk gave their soul to Asriel, making him respond to the locket [Something that will store Asriel and Frisk's souls there.] Until they see Chara, thinking that it's Frisk, until he started to recognize them. When Chara tells him that they will take his Soul, Asriel refuses, backing away from Chara.
After the fight with Chara, Asriel stands on Chara's grave, talking to Frisk, after a converstation, Asriel and Frisk [Inside Asriel] goes home.
The Way Home and The Memorial
These stories take place from the point where Asriel reunites with his parents-onward, seeing Asriel grow up on the surface.