MacBook – Connect 2 external Monitors with Belkin’s Thunderbolt 2 Express Dock HD

This is a really interesting question that gets to the heart of how display signals travel over Thunderbolt.

Your mac is capable of driving two "Thunderbolt displays". The special thing about the thunderbolt display is that it is capable of "pulling" the display signal out of the thunderbolt signal. This specifically is called "demuxing" (de-multiplexing). So technically, thunderbolt is capable of carrying two multiplexed display signals.

In contrast, when you use a displayport adapter on a thunderbolt chain, the bus driving that port from your mac is only capable of sending that one display signal since it is not being multiplexed on the wire.

The Elgato dock (or OWC dock) is capable of doing the same thing as a thunderbolt display, in that it is demuxing a display signal from the thunderbolt wire to provide an HDMI port. If you are using the HDMI port, you can't plug a displayport display into the other end of the thunderbolt chain -- there is no component to demux the other display signal. You COULD use a thunderbolt display in this position.

But, you have one other option! If you buy a SECOND Elgato device and add it to the thunderbolt chain, you now have a component to demux a second display signal to another HDMI port. Then you can use the two HDMI ports to drive whatever displays you want!

The reason why the dual setup isn't working is that you are using MiniDP & 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.