The answer depends upon whether the assignment occurred at expiration or prior to expiration (i.e., an American Style option).  At expiration, many clearinghouses employ an exercise by exception process intended to ease the operational overhead associated with the provision of exercise instructions by clearing members.  In the case of US securities options, for example, the OCC will automatically exercise any equity or index option which is in-the-money by at least $0.01 unless contrary exercise instructions are provided by the client to the clearing member. Accordingly, if the long option has the same expiration date as the short and at expiration is in-the-money by a minimum of the stated exercise by exception threshold, the clearinghouse it will be automatically exercised, effectively offsetting the stock obligation on the assignment.  Depending upon the option strike prices, this may result in a net cash debit or credit to the account.

If the assignment takes place prior to expiration neither IBKR nor the clearinghouse will act to exercise a long option held in the account as neither party can presume the intentions of the long option holder and the exercise of the long option prior to expiration is likely result in the forfeiture of time value which could be realized via the sale of the option.