How to sell an option against a long one in the account?

Procedurally, to sell short, all you need to do is specify your order Action as 'Sell' at the point you create your order. Note that we do not allow you to be both long and short the same security, so if you maintain a long position and enter a sell order, you will close out any long positions to the extent of your sell order and open a short position to the extent, if any, your sell order exceeds a long position. Please note that you must maintain a "Margin" type account with net liquidating equity of at least USD 2,000 for a short sale order to be accepted. Short sales are not allowed in "Cash" type accounts…

If you do not already hold a long position and enter a sell order, your order will be treated as a short sale (as accounts cannot simultaneously hold both a long and short position in the same security…

IBKR does support short sales in U.S. Corporate Bonds and U.S. Government Securities (e.g., Bills, Notes, Bonds) if the following conditions are met:

• You have a Margin account
• You have approved U.S. Bond trading permissions…

A Short Sell order will be accepted and can possibly trade if you have a margin type account and there is sufficient Available Funds to meet any change in the account's margin requirements. If If you do not want the ability to sell short, you would need to have a Cash type account as Cash accounts cannot borrow shares of stock to naked short sell, be short naked Call options or short non-cash secured Put options. Cash accounts are restricted from holding a short balance in any currency as this would also constitute a loan from IBKR…