  We present the experimental observation of a large exchange coupling $J
\approx 300$ $\mu$eV between two $^{31}$P electron spin qubits in silicon. The
singlet and triplet states of the coupled spins are monitored in real time by a
Single-Electron Transistor, which detects ionization from tunnel-rate-dependent
processes in the coupled spin system, yielding single-shot readout fidelities
above 95%. The triplet to singlet relaxation time $T_1 \approx 4$ ms at zero
magnetic field agrees with the theoretical prediction for $J$-coupled $^{31}$P
dimers in silicon. The time evolution of the 2-electron state populations gives
further insight into the valley-orbit eigenstates of the donor dimer, valley
selection rules and relaxation rates, and the role of hyperfine interactions.
These results pave the way to the realization of 2-qubit quantum logic gates
with spins in silicon, and highlight the necessity to adopt gating schemes
compatible with weak $J$-coupling strengths.
