  The implementation of high fidelity two-qubit gates is a bottleneck in the
progress towards universal quantum computation in semiconductor quantum dot
qubits. We study capacitive coupling between two triple quantum dot spin qubits
encoded in the S = 1/2, S_z = -1/2 decoherence-free subspace - the
exchange-only (EO) spin qubits. We report exact gate sequences for CPHASE and
CNOT gates, and demonstrate theoretically, the existence of multiple two-qubit
sweet spots (2QSS) in the parameter space of capacitively coupled EO qubits.
Gate operations have the advantage of being all-electrical, but charge noise
that couple to electrical parameters of the qubits cause decoherence. Assuming
noise with a 1/f spectrum, two-qubit gate fidelities and times are calculated,
which provide useful information on the noise threshold necessary for
fault-tolerance. We study two-qubit gates at single and multiple parameter
2QSS. In particular, for two existing EO implementations - the resonant
exchange (RX) and the always-on exchange-only (AEON) qubits - we compare
two-qubit gate fidelities and times at positions in parameter space where the
2QSS are simultaneously single-qubit sweet spots for the RX and AEON. These
results provide a potential route to the realization of high fidelity quantum
computation.
