Two monitors using Elgato dock and a thunderbolt device

I'm using an Elgato Thunderbolt 2 Dock which has a second Thunderbolt port and a seperate HDMI port. I have an HDMI display connected to the HDMI port and a LaCie HDD connected to the second thunderbolt port.

On that LaCie HDD there is an LG ULTRAWIDE connected via MiniDisplayPort to DisplayPort.

If I only plug in the HDMI device it works. If I only connect the DisplayPort-Device to the LaCie HDD it works. If I connect both displays only the one connected first will work but whenever I connect one of the displays the MacBook goes black for a moment and comes back with only one monitor recognized.

The Elgato homepage states that there needs to be another Thunderbolt device between the dock and the DisplayPort Monitor but my LaCie IS that device.

Why don't I get a signal on the second monitor?

MacBook Pro Mid-2012

OS X 10.9.5

Connection Diagram:

MBP => Elgato Dock (with HDMI connected) => LaCie HDD => DisplayPort Monitor