ElCaron, I think I'm experiencing the same problem as you, though I'm not sure what you mean by two different SPI interfaces. Could you elaborate a bit, perhaps posting your end-result pinout?
I'm using an ethernet shield, 20x4 LCD, and Arduino Mega. The LCD utilizes pins (7,8,9,10,11,12) and the SDCS at pin 4. I can run each component separately, though when combined together (specifically when I add the line 'lcd.setCursor(0,0)'), I fail to write anything to the SD.