Ø Back/Lay Field
Using this option you can back or lay the entire selected market using the odds set in the lay odds/back odds boxes using the stake that is in the box just above. If you have ‘Cancel remaining when matched’ checked then as soon as a selection gets matched all the other bets that were placed as part of the back/lay field option will be cancelled. This will not affect any other bets that you may already have on the market.
That's what it say in the guide but it doesn't say for a second bet. Maybe, you could just experiment and see what happens?