I've been happy with the various unit I've had and/or have still. D-Link and qnap. Synology is a popular bran in a similar position marketwise to qnap at least in the SOHO market.
There are many factors when purchasing a NAS.
The number of bays equates to the number of drives and this determines your maximum raid level. This is mainly subjective in the sense that only you can determine the level of fail over or redundancy you want to pay for? More bays equals more money. IMO, 2 bays are the ABSOLUTE minimum. There is also potentially a performance gain by being able to read from multiple drives simultaneously, but this is probably negligible for most home users.
Binding a drive to a particular NIC is not something I am aware of in any of the common manufacturers. Typically, a multiNIC unit is also a multi bay unit and people are looking to implement a raid strategy, not a split hardware setup. 2x single bay units would likely be better if that is truly what you want to do.
Value for money. Spending money on a faster processor and/or more RAM, or at least a unit that can be upgraded in the future, is often money well spent. My QNAP TS-563 with 16gb (upgraded from the original 2gb) ram and 5x 10tb hdds ticks along quite nicely compared to the older, slower TS-231P with 2gb ram for example. Both came with 2x 1GB NIC.
Now, more to your question about dual NIC. Depending on your setup and needs, 1 NIC may serve you well and you may never need anything more. A unit that can have a second NIC in the future might also be a good investment. For example, I added a 3rd NIC to my TS-563 (10Gbe).
With 2 NICs you can do port aggregation (if your router or switch also supports it). In theory, this doubles the bandwidth to the NAS, but there there are all sort of caveats and conditions that in a home environment mean you won't actually see much of a performance increase.
With 2+ NICs you can also "multi home" the NAS. In my case my TS-563 has 1 NIC connected to my primary router and acts as a web server via port forwarding. This sits in a middle area similar to a DMZ, but not quite, but effectively isolated from my internal secondary router and network. The 2nd NIC is for internal network connectivity. By isolating the networks and NICs on different subnets there is no way to breach the internal network from the internal and vice versa. Another similar scenario is to connect a specific PC via a dedicated NIC so no other systems use that access point effectively giving full bandwidth for heavy use such as large backups for regular file transfers. In my case, with 3x NICs, I run 1 NIC facing the internet, 1 NIC facing the internal LAN and the 3rd NIC direct to my workstation that performs all the backups, file management, etc.
With 2+ NICs you also have some redundancy. You may not need it now, but for a few dollars more you have a spare network connection in case one fails. Less likely, but a possibility. Again, having the option to add one in the future may be a good alternative.
Most decent NAS mid-range units come standard now with 2 NICs. If the model you are looking at only has one, it is likely an older model. Also, this usually also equates into a better processor and more ram.