How do I write a thank you card for a present and for coming to my party to a couple when only one came?

" THANK YOU for your kind gift and your attending my party"

you want to keep it simple.. and since you don't know who bought the gift.. or if it was both of the people, i wouldn't thank any one in particular. (dont name and names)

You know which person attended, so you thank them for attending and thank them both for the gift.