Connect 2 external Monitors with Belkin's Thunderbolt 2 Express Dock HD I need a little help here,

I have attached my MacBook Air Mid 2013, to an Belkin Thunderbolt 2 Express Dock HD.
On the product page it says that one can connect two monitors. The only condition is that one needs to be an Thunderbolt display. So far so good.

I attached one monitor with HDMI to the dock and another using the mini display port (which is compatible to Thunderbolt).
Unfortunately just the one with Mini-DP is working. If I disconnect the Mini-DP Monitor HDMI starts working.

Is there any advice why this is not working?

AND

Is there any option to make it work?



OSX 10.9.4 @ MacBook Air Mid 2013
Mini-DP Monitor: 2560x1080
HDMI-Monitor: 1920x1080
ref: EveryMac 

*Although Apple only reports that this model can support a single external display up to 2560x1600, Intel reports that this model can simultaneously support two external displays up to 2560x1600 "daisy chained" via Thunderbolt.

From Belkin's page on the Thunderbolt™ 2 Express Dock HD …


  "Note about dual displays: At least one display has to be Thunderbolt or Thunderbolt 2 technology-ready. For displays that are not Thunderbolt technology-ready, the Thunderbolt port is backwards compatible with Mini DisplayPort. The dock supports either HDMI or Mini DisplayPort, but not both. The dock supports only one display at 4K cinema resolution."