You have to rearrange pin wiring.
If you move 11, 12, 13 to the Mega's Hardware SPI pins, all <SD.h> programs are going to work. e.g. showBMP_Uno_kbv
Yes, you can share data bus pins. I would not risk sharing Control pins.
Life is safer if you re-route data bus to 22-29.
Remember to cut any ProtoShield tracks that you have moved.
David.