A friend recently asked me how I did it because she wasn't sure what the proper etiquette would be, and I hadn't really thought too much about it. I was wondering what everybody else did. She said she has friends who are a long-time couple but don't yet live together. She felt funny sending them separate single-addressed invitations, and one wasn't closer to her than the other, so she didn't want to just send to one with both names on it. I personally sent both people invitations.
Her other issue that hadn't come up for me was giving a single mom friend a plus one and also inviting her young daughter. Should she address it Megan and guest, and Ashley? Or Megan, Ashley, and guest? Or stick a Megan and guest invitation and an Ashley invitation in the same envelope?