As the error says - you cannot. Create another SSH key and use that for the second account.
Since cloning via SSH uses email@example.com it’s a single user, where each SSH key is. Therefore this is the reason why you have to use a separate SSH key for each account, otherwise how can it differentiate to which account to authenticate with.
If it was firstname.lastname@example.org and email@example.com for SSH, then you would be able to use the same SSH key, since it’s also providing a username that it would map to. But this is not possible, since SSH cloning is done via firstname.lastname@example.org and only this hence the requirement for SSH keys to be unique and not shared between accounts.