I have two Dell UltraSharp U2515H 25-Inch Screen LED-Lit Monitor's. I have a thunderbolt to display port going into 1 of the 2 monitors. On the 2nd monitor I have an HDMI going in. The dual screen works perfectly fine when plugged in for the first few wakes. After a couple days, I have realized that monitor #1 (thunderbolt) does not wake up and makes monitor #2 the primary making it #1. Sometimes unplugging both cables does the trick, but sometimes it requires a restart of the computer. Im not sure as to why that would be happening. This same scenario was occurring with the HDMI a couple months back but stopped now its doing the Thunderbolt one.