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I'm not so sure if that will work.
Most often in a small tank the mothers will eat the fry. If yo manage to prevent that, you will often need a seperate tank to raise the young.
And after a few generations you'll probably get a lot of mutated, shortlived and dull fry, because of inbreeding.
The guppies sold at one store are often already related to eachother.

Conclusion; it's not that cheap and easy...

but two females can't have babies

If a female guppy has been kept with fertile males while she was fertile then there is a big chance that she is or will get pregnant.
Guppies can store the sperm of the last male they mated with for quite a while. A few months I think. Basically after mating she can chose when she becomes pregnant.
The spermcells do get damaged over time, which increases the chance of deformities.


Heh, do that 2 or 3 times more and you'll be getting a lot of fry with two heads XD
I wish I could raise those to adulthood.

Well, it's in the nature of the female gup to want to mate with the prettiest male.
So as long as there are no predators the colors will keep getting brighter.

I immagine guppies being the victorian dandies reïncarnated.