Writing thank-you notes?

I would just send a general thank you to both. Don't single one out for coming to the party. That will make the other feel bad and if it wasn't their fault, that isn't very nice. Just say "Dear so & so, Thank you so much for the wonderful gift you gave us...." Expand on the gift, not the absence of one party or the other. Definitely save the Thank You cards for after the wedding. Keep it simple so you don't forget who you have sent a Thank You to already and who you did not. Good luck.