how do i join two apple accounts

Sharing purchased content with Family Sharing - Apple Support

You can not merge Apple ID accounts. It's easy enough to share one for purchases, but this is generally a bad idea as if something ever goes wrong in the relationship, there is no good way to split the content.

It's worth noting that most iTunes music is not copy protected. With Home Sharing, you should be able to import music to your iTunes library from hers.