Mac is mirroring my monitors through miniDP
The adapter in question only supports Windows 10 - it won't work on a Mac.

Connecting two displays via one of the Thunderbolt 3 connectors on the MacBook Pro 15" with TouchBar is usually done via daisy-chaining through the display. This is only supported by Apple for Thunderbolt displays - not DisplayPort monitors.

The cheapest  way to connect your monitors is to use two ports, as you mention. However it is not necessary buying two of those expensive dual-display adapters. Instead you can use a simple USB-C to DisplayPort cable. For example this one:

https://www.amazon.com/Cable-Matters-DisplayPort-Thunderbolt-Compatible/dp/B01J6DT070/ref%3Das_li_ss_tl?ie=UTF8&amp;qid=1484165396&amp;sr=8-1&amp;keywords=usbc%20displayport%204k&amp;linkCode=ll1&amp;tag=radianexuscor-20&amp;linkId=a1ef8c98169f6005d21382dca77d66e4&amp;utm_source=tr.im&amp;utm_medium=no_referer&amp;utm_campaign=tr.im%2FCable_Matters_USBC_DP&amp;utm_content=direct_input

Buying two of these ($15.99) will allow connecting the two monitors to your Mac. 

Hopefully you can return your adapter for a refund.