MacBook – ny thunderbolt device which supports daisy chaining and costs less than £100

Mini DisplayPort and Thunderbolt are essentially the same connection, just with different hardware controlling them. Thunderbolt is essentially a combination of Mini DisplayPort and PCIe. I assume this was done for backwards compatibility with older hardware that was Mini DP (so you could use an Apple Cinema Display on a newer model MacBook Pro, for example). Of course, Mini DP doesn't have the same power behind it (in that it doesn't combine with PCIe) as Thunderbolt so you can't daisy chain them.

Unfortunately, from what I have been able to see, and from my product knowledge working at an iStore, there is no Thunderbolt to HDMI, only Mini DP. The actual daisy chaining that goes on inside Thunderbolt devices is done within the device itself and would require a power source to power it. Since there is no power source to an adaptor, there is no point making a Thunderbolt to HDMI connector when Mini DP is (for all intents and purposes) the same thing. Apple also says here that if you have a Thunderbolt chain, the Mini DP connection should come last. This could be the reason for only one of your HDMI ports working as the built in one would most likely be positioned after the Mini DP connection.

Of course, that isn't to say there is no multi-output HDMI Thunderbolt peripheral (a single Thunderbolt cable to a device with multiple HDMI ports), it may exist as it is possible, I just haven't seen one. The closest I could find for you were these two devices, Mini DisplayPort to HDMI® (2x HDMI® Output) Converter w/ Audio and Zotac ZT-MDP2HD Mini DP to Dual HDMI Cable.

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!