In the UK we always recommend that only one USB socket is used at a time.

The front and rear USB sockets are not as separate as they appear and all share the same 5v rail on the CPU board (the front sockets are connected via a cable just behind the rear port.)

I have not tried your particular setup Dusan, but I can understand why it might cause a problem, not only do they share the same 5v line, but I believe they also share bandwidth with priority of transfer rate given to the rear port....but not 100% certain of this.