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.
On a Thermo board this works exactly as in Classic. The examples are from Thermo puzzles on this site.
Worked examples
A position from a real Thermo puzzle on this site. Step through it: the board lights what each step talks about, and the arrows show what rules what out.
Two digits, two cells, in a row
In row 3, look for two digits that fit in only two cells.
Where it fits
HardHidden Pair is a Hard technique: Hard Thermo puzzles are the first to need it.
The Classic Hidden Pair lesson teaches it on a plain 9×9 board. The Thermo overview explains the rule and the levels.
Hidden Pair in the other styles: Sudoku X, Jigsaw, Windoku, Arrow, Kropki.
Try it on a Hard Thermo puzzle. Next Step points it out when it is the move.