SPI is a bus. Just make sure every device on SPI has its own SS (Slave Select) line and you can have many devices connected to the same three SPI pins.
On a Mega you have indeed 50 (MISO), 51 (MOSI) and 52 (SCK). Standard SS is 53 but you need one of those for each device. Check the driver libraries that come with the respective devices on how to set this.