Belkin Thunderbolt 2 Express Dock HD dual monitor setup
The reason why the dual setup isn't working is that you are using MiniDP &amp; HDMI when dual monitors via MiniDP and HDMI are not supported, as noted in the product guide. You will have to use a Thunderbolt-ready monitor with a Thunderbolt cable to allow both monitors to be driven from the dock. 

The reason for this is that your dock is being driven by one Thunderbolt port, which does not allow for daisy-chaining displays (as far as the port on your computer is concerned) unless it is Thunderbolt 2 or the newest revision of DisplayPort. If you were to plug your monitor either into the spare Thunderbolt port on your computer (HDMI on the dock) or use the HDMI port on the computer (MiniDP on the dock) both monitors should work as a workaround.