If you will be having two USB sticks plugged into the USB hub, which is plugged into one of the B2's USB ports, I don't know how the B2 will be able to tell the difference between the two USB sticks. When you go to do an Export, I can imagine that various things might happen: files may be copied to one or the other or both, and maybe not always the same. Of course, you can experiment to see if it does what it does consistently.
I think the safer approach, and more solidly dependable way, would be to follow Paul's suggestion to use an ethernet connection, so you can devote a USB port to your backup disk.
One other question: When you have both your backup disk and childrens music disk plugged in, how will the B2 list those files? Normally, the files from a USB disk appear at the end of the list of files on the HDD (and with orange icons), but will you have to wade through all your backup files to get to the children's music? Or will the music from the two disks be intermingled? Again, I think you'll just have to try it and see.