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?

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OSX 10.9.4 @ MacBook Air Mid 2013

Mini-DP Monitor: 2560x1080

HDMI-Monitor: 1920x1080