If you intend to double-sell the item*, you'll need to edit the quantity to 2 in order to sell to the second bidder. Be aware that the quantity is displayed on the web catalog so you may want to edit the quantity just prior to (or even during) your event.
* Double-selling is when the auctioneer will conduct bidding as high as possible with the final two bidders, then ask both bidders to buy at that price if the donor will "agree" to donate another. For instance, two bidders remain and the last bid was $1200. The high bidder appears to have reached their "limit". Instead of asking the other bidder to raise it to $1300 - the auctioneer would offer to sell one to both bidders for $1200 each. A good auctioneer will make this look like a spontaneous idea - but it has usually been pre-authorized by the donor (and the auctioneer is aware of items that can be double-sold.)
To offer online pre-bidding on Live items, edit the item to type = Silent and mark the item for bidding pre-event online, paper at event. After the online bidding closes, edit the item again to change the type to Live and the high online bid as the starting bid during the live portion of your auction.