You can do it in same account via a calendar spread position. Short 10 ES H09 Long 10 ES M09 but that depends if it suits you strategy and if your broker is sensible the margin on that position would be 2 cents
shagi is right if you're trading do different contracts technically these are not offsetting securities... I didn't think this was what he was doing though.
In short, the answer to your question is: NO. TWS will not let you do this with one account. People have achieved what you want by using different accounts as you know. Not sure if this addresses your problem but you may also consider the following solution: Virtual Account ------------------- You could model a virtual account in your ATS that DOES have the ability to have both a long and short positions in a single instrument. The NET position would actually be transmitted to TWS from the virtual account. If both the long and short positions in your virtual account were equal, then obviously no trades would be transmitted to TWS. The virtual account approach has the added side effect of reducing commissions as it only trades the net differences. This can important if you have multiple strategies trading the same instrument in one IB account. BMe.