Women don't need to do much to keep a "backup" they have an endless supply of men chasing her. A "backup" can just be any guy she stays in contact with.
A man has to work a bit harder to prospect and keep "backups" but to me it seems sort of unnecessary to go out of your way to do it as some lame form of security. However if you are talking to multiple women and start dating one of them regularly, why not at least keep seeing the others? Until you agree to monogamy it's fair game. That said, I don't really have the patience to date multiple women anyway, it's too much work/money. You can still easily walk away without from a girl without immediate prospects to fall back on, so there's no real point in having backups unless we're strictly talking about having a constant funnel of pvssy, but I don't care about that either.