If something like that exists it would be a lot more expensive than the micro/miniSD to SD converters.
The micro, mini and regular SD all have the same pins, and work the same way (I think) so those "converters" are just a bit of plastic with a few wires in.
To actually convert between card formats, or use two cards as one would require onboard logic, and probably quite complex stuff.
You'd probably be better just buying a new card, considering the cost of flash memory these days.