Asking about how to apply to 2 open positions in same department in same group
At Australian universities, there are always selection criteria.  If the selection criteria are identical, you can submit identical applications.  If they are not identical, customize your applications to match the selection criteria.
Do address your letter to the correct professor.
If it is true, it would be fine to say in your application that you are equally interested in both positions.  It is quite likely X and Y know each other and are coordinating already.
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