I'm not sure I understand that one? I would think that the exposure (based on metering and the pre-flash) would be identical regardless of the timing of the flash relative to the shutter curtains. Obviously, you can adjust the ratio of flash to ambient in a number of ways (manual camera/flash, EC or FEC, max shutter in Av mode setting, etc.). But I don't think that just first vs. second curtain should make a difference.
Can you explain? Thanks!