  We measure singlet-triplet mixing in a precision fabricated double donor dot
comprising of 2 and 1 phosphorus atoms separated by $16{\pm}1$ nm. We identify
singlet and triplet-minus states by performing sequential independent spin
readout of the two electron system and probe its dependence on magnetic field
strength. The relaxation of singlet and triplet states are measured to be
$12.4{\pm}1.0$ s and $22.1{\pm}1.0$ s respectively at $B_z{=}2.5$ T.
