Hello Karkajou,
this seems to be a problem of the drivers for the Keyboards. Maybee they don't like each other.
Does the small keyboard work alone - is it recogniced if the other keyboard is not connected? First you need the small keyboard to work alone. If there is a driver supplied with this keyboard, it has to be installed first. When this keyboard is working, you could try to connect the two keyboards together. They should then be seen both by Hauptwerk.
I think, such small keyboards do not expect to be used with other keyboards simultanosely. I expect that a hub for connection won't help if it doesn't work with the two connectors offered by your computer.
Good luck - Martin (Dümig)