The process you're describing is commonly (and usually derisively) referred to as "bifurcation," because experience solvers will generally only resort to "guessing" when there are only two possible values for a cell.

Strategies for solving sudoku puzzles are many and varied, but if a puzzle can only be solved by "guessing a number" rather than logically working out where a number goes or which number goes in a cell, it's probably a pretty bad puzzle.