Keep the pregnant guppy in there until she doesn't look big anymore. It might take a couple hours or even a day before all the fry are out. Keep the fry in the tank for at least 3 weeks or until they are big enough that they won't get eaten in your main tank. Congrats in your guppy fry!
thanks for all the advice.... The one with 2 heads has been alive for about 12 hours so its doing ok I guess. If it is still alive tomorrow I will post a picture. Just a few other questions (sorry for all of them). Would it be better to divide the tank and put them in one side and leave the others in the opposite side after they have grown out of the breeders tank because there are over 30 in there at the moment. And what type of food is best for them?
You never have to apologize for asking questions You can divide the tank if you like to keep the babies safe. However, keep an eye on your ammonia, nitrite and nitrate levels as that's a good many fish in a 20l tank. If you have the space and the means, another tank for the fry might be a good idea. Something to think about. As they begin to grow the waste they produce is going to grow too causing ammonia issues. Ammonia can be fatal to your fish.
There is a food out there called First Bites that I've seen mentioned. However, Guppies I had many years ago, I just crushed up the flakes really fine and they all did very well.
if ur lil 2 headed fishy survives, it may best to keep it separate from the others so it gets enought to eat and doesn't reproduce. other than that comment, every one else has very good advice! good luck!