As I understand you can use multiple serial ports at the same time, but you have to initialize all of them for example:
To use the serial port 0:
Serial.Begin(XXXXX); //Enter the baud rate you want to use instead of the XXXXX
To use the serial port 1:
Serial1.Begin(XXXXX); //Enter the baud rate you want to use instead of the XXXXX
In regard to using the SD port is a little bit different because you use the SDIO, if you follow the link I will post bellow you will find a post about reading and writing on the SD Card, it's an Edison post but Ihink it should work on Galileo as well.
If you have any other doubts don't hesitate to ask.