But, it seems that dual-port NVMe really does involve bifurcating the x4 link into 2 x2 links, each of which can be connected to a separate controller. This isn't a terribly good explanation, but seems to confirm that dual-port is really a feature aimed at high-availability & fault-tolerance:
March 31st 2016. It’s last day of Q1 and it’s full of surprises. If you missed the announcement, take a chance and spend a minute reading press release. I’m proud today; we at NSG (Non-volatile Memory Solutions Group) have just released new products, based on very new to the industry...
It's a weird feature, because you generally cannot connect a drive to multiple hosts for the simple purpose of data-sharing. I've heard of people doing this decades ago, using SCSI drives, but filesystems and the host I/O subsystems of the day were much simpler.