Help me navigate the dongle/dock situation please!
You will need an external GPU driven by thunderbolt 3 if you want to output two HDMI signals from one usb-c port. (Something like this https://www.amazon.com/gp/aw/d/B075RTTFDJ/ref=mp_s_a_1_9?ie=UTF8&amp;qid=1512722145&amp;sr=8-9&amp;pi=AC_SX236_SY340_QL65&amp;keywords=external+gpu+thunderbolt+3)

If you just want to have a mirror from one to another, I think you can just use some HDMI doubler but I Donny think you will keep the same image quality (https://www.amazon.com/gp/aw/s/ref=is_s?k=hdmi+doubler)

The best, easiest, and cheaper way to do what you want is just to use two ports as HDMI output on your Mac.