It would be scored as the dog having retrieved two marks.
I had a dog actually do this at a test. Only, of the two birds she picked up, one happened to be a rotten, nasty old rotting bird that was on the shore en route to a pretty difficult memory bird.
It was actually quite impressive. She was on a really nice line down the shore and took a hard left and started hunting. I sat patiently thinking maybe she was caught in dragback... Instead, within a few moments, she picked up a bird...
I thought "oh, well, not her fault a bird was out there, judges will score it and we'll be fine"...
WELL.... Instead of turning around she PROCEEDED to shore, up and out, WITH THE FIRST BIRD IN HER MOUTH... and then I thought "what is she doing???" And then I thought "I wonder if this could be a switch???"...
She went right to the original memory bird, took a few seconds and got both birds in her mouth and returned with both of them.
I was really impressed. Like she KNEW that gross bird wasn't the one to get and she knew she needed to get the one I asked her to get...
Anyway... Plenty of dogs can fit two birds in their mouth one way or another. I guess it helps when one is a rotten skinny sogging wet bird.
-K