It's more that two copies of the same bitcoin will be created (one with segiwt and one without). While it is needed to activate segwit, there should be a way created that completely destroyes the other bitcoin.It probably will happen that it will break into two blockchains, as I'm not sure how much hashpower will be behid the non-segwit bitcoin but all it takes it one core to be mining the currency (on an incompatable) segwit release and then thre will be the separate currency creasted (much less power and userbase behind it but still a separate chain) - and it doesn't take much to get that second chain running.