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…

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…

In the IBKR trading platforms you simply enter a buy or sell order and the IBKR system marks your order as an opening buy or sell or a closing buy or sell. As an example, create a short sell order in the IBKR trading platform and IBKR checks the account's position and marks the order as an opening or closing order.  Note that IBKR does not allow clients to simultaneously maintain both a long and short position in a given instrument, any order to sell (buy) will only result in a short (long) position to the extent the order quantity exceeds any existing long (short) position held and which would then be closed by execution of that order…