  We present a new and simplified two-qubit randomized benchmarking procedure
that operates only in the symmetric subspace of a pair of qubits and is well
suited for benchmarking trapped-ion systems. By performing benchmarking only in
the symmetric subspace, we drastically reduce the experimental complexity,
number of gates required, and run time. The protocol is demonstrated on trapped
ions using collective single-qubit rotations and the Molmer-Sorenson (MS)
interaction to estimate an entangling gate error of $2(1) \times 10^{-3}$. We
analyze the expected errors in the MS gate and find that population remains
mostly in the symmetric subspace. The errors that mix symmetric and
anti-symmetric subspaces appear as leakage and we characterize them by
combining our protocol with recently proposed leakage benchmarking.
Generalizations and limitations of the protocol are also discussed.
