Let's say you are doing something crazy like trading an expensive contract like HSI or DAX. You can only afford one contract plus reserves for swings. You are long in your system. Now you want to go short. If you enter a placeOrder for 2 SELL, IB will complain about, not enough margin. Is the proper way to send Sell 1, wait on status, and then sell an additional contract?. It seems like IB could handle this condition, but I remember a while back (several years), when this was a problem.
Talking futures, here. You are trading 24/7 and are at full margin. Just need to reverse, and not run afoul of margin.