Hi Theerapun
1)Entrainment to a binaural beat is far more effective if it starts at normal wakefulness, because that is the brain state we are normally in before a session, and pulls us down gradually to a slower wave. If you made just a straight line at 8hz, it will still take time before you are entrained to that level, so you might as well accept this time period as a time for controlled entrainment, controlled by Bwgen as a sloping line over a 20 minute time period.(a gradual slowing of the brain waves to the level you want). A gradual pulling down of your brainwaves by a gradual sloping line might even be more effective and faster than if had a straight 8hz line.
So it is wise to start from 13hz whether you are going up or down, because this is generally normal wakefulness.
2)With multiple voices, set one binaural beat frequency to 8hz and the associated pitch to 100hz, then set the other binaural beat frequency to 40hz and the associated pitch to 200hz or higher. This setting of the different binaural beat frequencies to different pitches gives your brain something to grip onto, if both were the same pitches it would be difficult for your brain to distinguish between binaural beat frequencies, and is likely to thwart your effort to entrain your brain to either 40hz or 8hz instead of doing them together.
Your brain has multiple brainwaves beating through it at the same time, naturally, and these can be entrained with multiple voices with BWGEN.
3)Have no fear, create your own preset, in the menus at the top select new preset. Create two voices. Start the first voice at 13hz dropping down to 8hz over 20 minutes. Make the second voice also start at 8hz, but going up to 40hz over 20 minutes.
Play the preset and listen for 20 minutes, Bwgen will continue playing after 20 minutes, one voice is remaining at 8 the other at 40, as it shows on your monitor. Continue listening, the longer you listen the greater effect. Try listening to the whole preset for 40 minutes altogether.
Good luck,
Grant.