Can one buy an Ethernet and HDMI Thunderbolt adapter?
Yes - there are quite a few docks to connect one cable to a Mac and have gigabit ethernet as well as HDMI out (and some even support 4K resolution on the HDMI).


Elgato https://www.elgato.com/en/thunderbolt-2-dock
Belkin http://www.belkin.com/us/p/P-F4U085/
Sonnet http://www.sonnettech.com/product/echo15thunderboltdock.html
OWC https://eshop.macsales.com/shop/Thunderbolt/Dock/OWC/Thunderbolt2-Dock/


I've linked to the top of the line thunderbolt 2 versions of the docks. If you don't need the fastest speed look for older models of 10 Gbps thunderbolt as refurb/used or still new stock which are discounted in price from the faster 20 Gbps models.

It's far cheaper to buy two single use adapters, though - the electronics are smaller and less complicated than needed for a full dock that takes one cable and splits into multiple functions.