FPP with Cyclone IV GX and multi-voltages

I'm using FPP configuration mode with Cyclone IV GX (484pkg). Data 5-7 is on bank3, which is also my DDR2 bank and is at 1.8V. All other config pins are at 3.3V. My probem is that I must set the MSEL pins according to the IO voltage of the config pins, but I have two config IOs voltages. Should I set MSEL for 3.3V or 1.8V ? What's the difference internally ?