Before you can get out of a trade, it's either you complete the trade or cancel it, so just don't assume they will allow you into another trade when the one you've started is still pending or inconclusive, these measures you see the exchanges take into effective use were just to avoid mixing things up and user trading a particular coin twice which may be a loss to them because they will be responsible for that order, i will advise OP to move this thread to trading discussion board for more information on this area of discussion.