If you're adding a guarantee to a adult/guarantor record (using the 'Add a guarantee' button) then the guarantor's details should pre-fill the guarantee alternate contact fields.
Actually I think this would be better handled in bug 22534 , as library staff would have more control over what fields are pre-filled in guarantee record from guarantor. So will close this ticket and work on 22534 instead. *** This bug has been marked as a duplicate of bug 22534 ***