There's no reason not to double-snipe if you're trying to counter another sniper's expected last-second bid. Just set your lower bid five seconds before you set another, higher bid. Say you want to bid $10 for something but think it might take $15 to hold onto it. Set the bid for $10 to be submitted 10 seconds before the auction closes and another bid for $15 on the same item to be submitted 5 seconds before the auction closes. If $10 turns out to be enough, then you haven't lost anything because you can't outbid yourself on eBay, so your $15 bid is ignored. But if somebody bids $12 in the last few seconds of the auction, you'll be glad you put that $15 bid in there.
Of course, you could also put your $15 bid in by itself 5 seconds before the auction ends since you're willing to go that much, but if you like to put a little excitement in your bidding then double-snipe.