Simultaneous Approvals cause one of the node to fail and throw exception error "Unable to write output document"

Once the first Approver opens task/view the document, a lock is placed on that file until that person is finished, this maintains document integrity. If the exception happens, I would recommend handling it then redirecting the Approval back to the document, in which time the lock would be released should the other Approver complete the Task rather than timed delays.

The node you mentioned as Generation of Memo is not visible in the screen shot. Anyways why don't you try putting a timer each after the UI. Wouldn't that help?

How are the other tasks halted? Are you using Exception or is the process getting terminated which in turn takes the task away from those other users?
In reality (a) you should be using Exceptions to manage this and (b) You need to educate the users that the task will not be available for them to complete (assuming they have it open) because its gone into exception. If you insist on this way, then the other thing you can do is to NOT terminate the task but have an XOR gateway immediately after the task that checks if any other task had been completed. Use a process variable to handle this. If that be the case, then show a follow-up screen (activity chained) to the user indicating this task has already been completed. You can then put a timer + rule based exception to automatically terminate the sub process when the parent process is done.