Pair hidden single
A letter/digit pair that fits in only one cell goes there
How to use it
Ask the grid-wide question: every unused pair must be placed exactly once somewhere. For a pair, say C3, collect its homes: the empty cells whose letter candidates admit C, whose digit candidates admit 3, and where C3 is neither placed nor blocked. If the pair has exactly one home, it goes there.
Unlike the per-layer hidden single this looks along both layers at once: a home must be reachable by C and by 3. Scan pairs whose letter is nearly placed — a letter with most of its cells pinned has little freedom left for its remaining pairs.
Why it is sound
By orthogonality every pair occurs exactly once in the grid. If every cell except one excludes the pair (its letter or digit is impossible there, or the pair is used or blocked), the remaining home carries it in every solution.
A worked example
Ask the grid-wide question for the pair D3: which cells could hold it? In this position D still survives at eight cells and 3 at six — but exactly one cell, r4c4, admits both halves at once, so D3 goes exactly there. The placement then removes D from row 4 and column 4, and 3 likewise, in one stroke. Pair hidden singles fire most often late, when most of a letter's cells are already pinned.