Eternalism
The model of time in which outcomes ruled out by a choice can go on existing as other real possibilities.
TIME · POSSIBILITYFULL SPOILERS
How it differs from presentism
Presentism counts only the lived present as real. Eternalism does not wholly abolish the outcome that did not happen; the Void can put those possibilities in contact.
Operator and Drifter
The two characters are not different ages of one person but separately lived outcomes of the same beginning. Neither one's existence makes the other a dream or a copy.
The limit
Not every imagining automatically becomes real, and no death is undone on the excuse of another branch. For contact to happen, the story has to show a specific Void or temporal link.