To do this you need to "trunk" (also called "bonding" or "link aggregation") your two NICs. To do this you need:
- NICs that support this (most common are LACP or 802.3ad)
- An OS that supports this
- A switch that supports this
Windows does not support this out of the box, so you will need to use latest manufacturer drivers for your NICs, and follow their instructions for enabling this functionality.
I don't think the DGS-1016D supports link aggregation though, so you might have been struck out on the first hurdle.
The other option is to run different services on the different NICs, so if your issue is with multiple concurrent file transfers, run half over the IP address of NIC1 and half over the IP address of NIC2. This should give you a similar overall result.