You only need to tag animals to either:
- meet pedigree requirements
- meet legal requirements
- allow you to manage you herd
I presume it is to meet pedigree that is the reason, as unless/until you need to move her from your premises, she needs no further ID, and I suspect you'll not wish to tag her just so that you know who is who, and if so, she would only need one tag.
Question - who is saying she needs double tagging? If she was bred after Jan 2011 (I think) then she should have been notched rather than tattooed, so is this to now meet BPA reqs? If so, do you have direct permission from the BPA to do this, as their permission is required? And have they said what with in terms of ID? Make sure you are clear on why you are doing this - probably you are, but there is much confusion over pig identification around, and want to make sure you are doing it because it is really needed!
If so, I would use button tags, and it is quite easy. Just do it whilst she is eating. No it is not worse on older than younger pigs, we do it sometimes on older pigs for management reasons. Just lift the ear, position pliers in the centre of the ear and close quickly. If you are doing two tags, maybe leave a couple of days between goes !