Each system manufacturer has discretion when designing their Thunderbolt 3 systems as to how many DisplayPort video outputs are routed out via a single Thunderbolt 3 port, however their documentation as to the specific capabilities each system offers is often not clear.
Some manufacturers like Dell and Apple support two displays via a single Thunderbolt 3 port, however in my experience no currently available Intel NUC with Thunderbolt 3 can drive two displays via a single Thunderbolt 3 port. The currently available Intel NUCs are limited to one display only via their Thunderbolt 3 ports. Most recently confirmed on the seventh generation Intel NUC7i7BNH with a Plugable Thunderbolt 3 Dual Display HDMI Adapter (TBT3-HDMI2X-83). Full disclosure, I work for Plugable Technologies.
If you connect a Thunderbolt 3 Dual Display adapter to one of the currently available Intel NUCs with Thunderbolt 3, only one of the adapter's two outputs will work. This is an internal system limitation (not a limitation of the adapter) and this behavior cannot be altered via firmware or software.