This would do what you want, with the A male being a redundant connector.
http://www.ebay.co.uk/itm/5Ft-USB-2-0-Type-A-Male-TO-Micro-B-Mini-B-Male-2in1-Combo-Cable-Cord-New-/390552210557?pt=US_USB_Cables_Hubs_Adapters&hash=item5aeeb9cc7d
Although the cable is 5ft in length - based on the photo, the maximum distance between the mini and micro connections appears to be about 1ft.
I suspect the reason no one produces such a cable (that you want) is that the devices you want to connect using mini & micro usb will not communicate with each other (when so connected).