I've had to use twin charging on occasion, although I always put that down to my cheap chinese charge cable starting to go south, and shortly afterwards they've proved to need replacement. I don't see why it should work to be honest; the 5V sources must be united before they hit the actual charge chip, and out from there toward the battery there's only the charge line, and the sense line (ignoring ground for now). The 5V from mini-USB must go through more complex transforms, since it can also work as an OTG port, so must have a charge ladder to get than 5V from system 3.3V, but presumably that's switched out when it's acting in client mode, and I guess just presents the near 5V from USB to the charge chip.
It can't hurt, but I don't understand why it works if it isn't cable troubles.