well you CAN keep them together in a large enough tank..
realistically people are exaggerating when they say they will find each other and kill each other! ..... sure its possible they can die but its not likely (in a big enough tank of course if its a small tank different story)
BUT the fact of the matter is bettas like to be alone and if you do have multiple bettas in the one tank what WILL happen is you will have stressed bettas, obviously damaged bettas because they will fight but not necessarily to the death, the loser will run away and be timid whilever the winner is still in the tank.
so not only do you have stressed injured (which also increases chances of catching disease) bettas, you will have some very timid bettas who will do nothing all day and have a shorter life span than others due to constant stress and injury, this timidness takes away the very thing that makes bettas unique and enjoyable to have, THEIR PERSONALITY, if they arnt happy (which they wont be) they wont have a personality either
so no, please do not do it