  The complex collisional properties of atoms fundamentally limit
investigations into a range of processes in many-atom ensembles. In contrast,
the bottom-up assembly of few- and many-body systems from individual atoms
offers a controlled approach to isolating and studying such collisional
processes. Here, we use optical tweezers to individually assemble pairs of
trapped $^{85}$Rb atoms, and study the spin dynamics of the two-body system in
a thermal state. The spin-2 atoms show strong pair correlation between magnetic
sublevels on timescales exceeding one second, with measured relative number
fluctuations $11.9\pm0.3$ dB below quantum shot noise, limited only by
detection efficiency. Spin populations display relaxation dynamics consistent
with simulations and theoretical predictions for $^{85}$Rb spin interactions,
and contrary to the coherent spin waves witnessed in finite-temperature
many-body experiments and zero-temperature two-body experiments. Our
experimental approach offers a versatile platform for studying two-body quantum
dynamics and may provide a route to thermally-robust entanglement generation.
