The problems with this are as follows:
1) The females will likely fight. They would be trying to impress the male by showing which one of them is stronger. This could either damage them very badly, or kill them outright.
2) While the male is busy with one female, the other female will be eating the eggs produced. Which kind of eliminates the whole point of spawning them, since a lot of eggs are bound to be eaten.
3) The girls can tag-team and attack the male, leaving you with an all-out brawl on your hands, and three very damaged Bettas.
I really wouldn't recommend it.
One thing you CAN do, though, is put a female in each of two different jars within the tank, and see which one the male flirts with most, and which one flirts back the most. But do NOT, do NOT release both females in with the male. Too many bad things can happen.
