I have a Linear HUSBZB-1 working well for Zigbee devices.
- Can i improve the Zigbee mesh by adding another of these USB devices?
- How can i configure the second device to be a secondary controller?
Assuming zigbee works like ZWave, you would be better off adding more mains powered devices to your existing controller. If not then I’ve got nothing.
Is there a way to see what devices are participating as controllers in the zigbee mesh?
Yes, it is meshed. Adding mains powered devices somewhere between your last reachable device and the not reachable device should do the trick. A simple HUE lamp would be enough (it must have power all the time, but can be off).
Almost every mains powered device works as a repeater. As far as I know there is no way to check this in OH, you have to read the manuals of the products to be sure.
The device attributes will say if its a coordinator, end device or router.
I have a very similiar situation, and solved it by plugging a simple Osram Lightify Power Plug in an otherwise empty mains outlet in the living room. The outlet can still be used, and i have nice ZigBee repeater 
A better looking solution will be converting my kitchen lights to Hue, but this still needs approval from the Lady of the house 