Using 2 Ethernet ports at once?

If you run a cable directly between the two, and make sure that the NAS is accessed over that cable (for example by using an IP address specifically available over that link), you can make sure that the Mac and the NAS has the full gigabit speed available to them. If you have other devices accessing your NAS at the same time, this will somewhat improve the speed (given that the NAS does not have other bottlenecks).

It will however not give you combined speed of both interfaces.

If you want that, you want to go ahead and setup link aggregation. This way you can "bond" the two ethernet ports into a single virtual network port. The Mac will then have a higher capacity link to your switch. This can increase bandwidth capacity when you community with multiple different devices connected to that switch. If you also similarly connect the NAS using link aggregation with two or more interfaces, you can get the combined speed of both ethernet ports of the Mac when accessing the Mac.

NOTE: This requires your switch to support link aggregation (notably in the form of LACP). You need to set this up on your switch.

To set up link aggregation on your Mac, you do as follows:

1. Open System Preferences > Network

2. Click the cog-wheel below the list on the left, and select "Manage Virtual Interfaces"

3. Click the plus button and select "New Link Aggregate"

4. Select the two ethernet ports and click "Apply"