My setup: Late 2013 rMBP 15" w/ NVIDIA 750M clamshell mode Apple Thunderbolt Display @ best for display Dell UP2414Q (MiniDisplay Port, running on DisplayPort 1.2) @ best for display On 10.9.2, I could use both monitors. Since upgrading to 10.9.3, I can only use the Thunderbolt Display OR Dell UP2414Q. I cannot use both at the same time. I've verified this behavior by: 1) Plugging in both displays 2) Unplugging the Dell - the Thunderbolt Display becomes active. 3) Plugging in the Dell and unplugging the Thunderbolt Display - Dell becomes active (No reboots/logouts occurred between the steps, and I've verified that both Thunderbolt ports are working.) I've already tried reseting the NVRAM and SMC. Any ideas?