[Buy and Sell deals at the same time on the same market]

I use browser, at the bottom of the sections "Deal" or "Order" you can choose to place another order/deal to off set the existing one "Net Off" or to add it in parallel "Force open". 

[Buy and Sell deals at the same time on the same market]

When i try to open a second deal on the market in the opposite direction it cancels (or partially cancels) the original deal. So I can have only SELL or only BUY deal on one market. IG help and support had a section about orders and limits where it tells "You can open two separate trades in the same market in the opposite direction. " Could somebody please clarify how to do it so I have two trades in the opposite direction on the same market at one time?