  Systems hosting multiple giant planets are important laboratories for
understanding planetary formation and migration processes. We present a nearly
decade-long Doppler spectroscopy campaign from the Keck-I telescope to
characterize the two transiting giant planets orbiting Kepler-511 on orbits of
27 days and 297 days. The radial velocity measurements yield precise masses for
both planets, which we use to infer their bulk heavy element content. Both
planets contain approximately 30 Earth masses of heavy elements, but their bulk
metallicities (i.e., the ratio between metal mass and total mass) are
drastically different ($0.86 \pm 0.04$ and $0.22 \pm 0.04$ respectively).
Envelope mass loss cannot account for this difference due to the relatively
large orbital distance and mass of the inner planet. We conclude that the outer
planet underwent runaway gas accretion while the inner planet did not. This
bifurcation in accretion histories is likely a result of the accretion of gas
with very different metallicities by the two planets or the late formation of
the inner planet from a merger of sub-Neptunes. Kepler-511 uniquely
demonstrates how giant planet formation can produce strikingly different
outcomes even for planets in the same system.
