When two digits can go only in the same two cells of a house, those cells must hold them, and their other candidates can go.
Scan the windows as well as the boxes: a hidden pair can sit in one.
Worked examples
2 positions from real Windoku puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the arrows show what rules what out.
Two digits, two cells, in a column
In column 6, look for two digits that fit in only two cells.
Two digits, two cells, in a window
Look through the lit window for two digits with only two places.
Where it fits
HardHidden Pair is a Hard technique: Hard Windoku puzzles are the first to need it.
The Classic Hidden Pair lesson teaches it on a plain 9×9 board. The Windoku overview explains the rule and the levels.
Hidden Pair in the other styles: Sudoku X, Jigsaw, Thermo, Arrow, Kropki.
Try it on a Hard Windoku puzzle. Next Step points it out when it is the move.