Of course I tried it out, before I posted this question. IMHO the problem is, that there is only 1 predefined mountpoint and the system tries to mount both usb-devices to the same mountpoint. Is there nobody with an usb-cardreader to test it too? I read in the net, that there are tablets where it works. So maybe there is some configuration missing.
If you have rooted your device this should´nt be a problem.
With root access you can write your own script to handle the mountpoints or use an app. I´m using https://play.google.com/store/apps/details?id=eu.chainfire.stickmount&hl=de and it works perfect for multimble devices on my powerd 4port switch. Just tested before i was replying to your thread.