With IB's native default order, you cannot place an OTO order such as: if your order#1 (BUY 500sh. of PQR stock at limit price of $12.00) completely gets filled, only then place the order #2 (BUY 500sh. of XYZ stock at limit price of $11.00). Suppose you do not want to use any 3rd party tools to accomplish it, then you can use IB's API. Fidelity, Ameritrade (including Izone), etc. have this type of orders directly.