I am using Sakura on ym M3 Real, and have an old G6 Lite in my slot 2. How can I boot from it? If I remove the M3real, I can boot to the GBA part of the G6 lite, but does anyone know how I can boot to the NDS part?

The M3 Sakura FAQ said:Q: How do I boot into Slot 2 with Sakura?
A: To boot into Slot 2 in DS mode, hold down B while booting. To boot into Slot 2 in GBA mode, hold down SELECT instead. This function is built into the flash cart itself; it is not a feature of the firmware, which means that it will work whether you have the M3 Real or M3 Sakura loader installed (tip by strata8). If you have an EZFlash 3-in-1, you can use GBA Exploader by Rudolph.
Note that you can't boot into GBA mode with a GBA Expansion Pack. If you try, you'll just get two white screens. You really wouldn't have a reason to boot into GBA mode anyway, because with a GBA Expansion Pack you can load GBA ROMs in Sakura simply by tapping/sliding them (much in the same way you would load an NDS ROM). More info on booting into Slot 2 and troubleshooting tips here, here and here.