  We report the simultaneous operation and two-qubit coupling measurement of a
pair of two-electron spin qubits that are actively decoupled from quasistatic
nuclear noise in a GaAs quadruple quantum dot array. Coherent Rabi oscillations
of both qubits (decay time $\approx$2 {\mu}s; frequency few MHz) are achieved
by continuously tuning the drive frequency using rapidly converging real-time
Hamiltonian estimators. By state conditional exchange oscillation measurements,
we also observe strong two-qubit capacitive interaction (> 190 MHz). We show
that the scaling of the capacitive interaction with respect to intra-qubit
exchange energies is stronger than the bilinear form, consistent with recent
theoretical predictions. We observe a high ratio (>16) between coherence and
conditional phase-flip time, which supports the possibility of generating
high-fidelity and fast quantum entanglement between encoded spin qubits using a
simple capacitive interaction.
