J.E.
My kids are not twins, but their birthdays are 11 days apart, so I have occasionally (twice so far) held one party for the two of them. (boy and girl, 3 yr difference).
I let each child invite about 6-8 kids (my older son usually invites less, my younger daughter wants more) and they work out the numbers between them. That said, I only put one child's name on the invite and let gifts come for the person who did the inviting (even if my daughter ends up with more gifts). If the invite is going to family or close family friends, I mention both children.
I've had parents tell me on the day of the party that I should have mentioned the party was for both kids, but I feel like they shouldn't have to get gifts for both my children if their child is only friends with one.
I also only let my children play with one gift after the party and they don't get to open another until they write the thank-you note for the first gift, and the next one they want to open. Each time they want to play with the next gift, they write the thank you note. This way, writing the notes are spread out over a couple of days, and all are finished within a week. This works for Christmas too...