No, sorry.

NSB-25 does not support external USB hub.
Only direct USB drive connection is allowed.

I got my NSB-25 finally, and was able to test this.

The NSB was able to recognize two USB drives on a powered USB hub, but only one at a time. It seemed more like the NSB just had to pick one of the drives because of GUI space issues (only 3 drive icons on screen), but when I'd connect/reconnect the USB drives at the hub, I could get both to be reconnected.

Is it possible in a future firmware update to, if only one Bridge Pellet is installed, to get two USB drives working?

It is standard procedure (in the US, at least) for producers to bring two USB drives (one primary, one backup) to a small shoot and have the DP or AC copy to both drives so the producer has the safety, not the DP.

If the NSB can make two 2 USB copies at once, it becomes much more useful.

Thank you for the suggestion.
For NSB-25, the internal firmware architecture is designed based on single USB drive with two bridge pellet. So, changing it is something like re-writing the whole firmware. We will consider that when we do next model like NSB-35(Although we are still in R&D stage).
By the way, do you know Bridge pellet itself is USB drive?
Remove the bridge pellet and check the rear side. You can see USB3.0 type-B connector for USB connection to computer.