When people are trying to double leg takedown you (like Habin in the beginning of the chapter), things you can do:
1. Evade --
Best course of action, though connecting this with a counterattack would require high agility and coordination. But, unfortunately, only inexperienced people will attempt to double you from a distance like some sotra bull.
2. Push down their body (like the impostor dude here) --
Not preferable since that creates a stalemate: your legs are still grabbed, and most of opponent's soft spots are covered. Still, better than nothing, and has the largest window to perform. Chances are you'll do that anyway, but probably too late if not trained and/or prepared.
3. Grab their body while they're rushing and guide it slightly sideways --
Good, solid method that requires practice. This way you both create some distance and throw your opponent on the ground.
4. Kick their head or shoulder --
a 50/50 risky move. If you succeed your opponent will receive quite some shock, open for your next move. But done incorrectly or just poorly, and it will lead you to loosing your balance, and falling much harder than otherwise. On top of that your leg is most likely to be grabbed by them. Overall not recommended.
5. Grab their head with both of your hands and twist it sideways not loosing the opponent's inertia --
probably one of the best techniques that works perfectly due to body mechanics: neck manipulation is a pressure point, so their body will not only instantly loose all power, but will also follow the head twist to avoid neck damage. Your use of their inertia makes it extremely hard to resist you. It requires quite a bit of training and is still somewhat risky, but much less than the previous method. Opponent ends up slamming on the ground with next to no window to perform any kind of ukemi (fall damage control) in a semi-spinning fashion.
Obviously, there is a somewhat high chance of neck damage, especially if the opponent tries to resist. So don't do this in not a life-or-death situation.
Anyway, don't try to double people anyway, unless you know what you're going to do next.