We recommend the exploration of joint survey strategies that combine the two topranked survey endeavors of the 2010 Decadal Survey, the ground-based Large Synoptic Survey Telescope (LSST) and the space-based Wide Field Infrared Survey Telescope (WFIRST). By the time WFIRST launches, LSST will have been in survey mode for several years already and will have built up substantial survey depth and area. The science collaborations of LSST will know what limits the precision of their core science cases and what type of space observations are most valuable to overcome key systematic uncertainties. WFIRST, with its broad range of capabilities, will be an ideal partner observatory to LSST at exactly the right time. In this white paper we outline two concepts for joint survey scenarios: The first is a 5-month, wide WFIRST survey in one band that would cover the full LSST survey area. This 5-month endeavor would provide high-resolution imaging for >95% of the LSST Year 10 weak lensing galaxy sample. The second scenario is a 1.5-year wide WFIRST survey in one band that would cover the full LSST survey area. For this second scenario we explore the joint LSST+WFIRST science return on cosmic acceleration from a joint weak lensing and galaxy clustering analysis. The combination of space-based resolution, color information from the ground, and infrared coverage from space over 18,000 deg2 would enable a new level of precision for the existing core science cases of both experiments. It is now timely for WFIRST and LSST to set up a joint survey optimization and systematics mitigation effort that maximizes the science return for the community from the next decade’s flagship experiments in survey cosmology. 1LSST DESC is co-signing this white paper to endorse its content, having reviewed it following its internal management process for paper endorsement.