Merry Christmas, I just made you this Bookmarklet that does both:
Technically it doesnt add them at the same time, it does one after the other. It takes advantage of Pinboard.in's
next= url parameter that I added to the regular bookmarklet. In it I specified it to go immediately to the delicious add url.
So, when you click on it, it will bring up the popup window for you to add the bookmark to Pinboard and as soon as you hit save, the popup window changes to a delicious popup window where you can save it there as well.
It will populate the url and title automatically, but since this is happening before you enter your tags and note, they are not passed on and you'll have to type them in again.