It's very possable. Just because one looks more black than the other doesn't mean they are. They are both equally white and black.
Many people that may appear all black, but may have a lot of white in their genes, and vise versa.
You have to look at other things rather than skin color. You have to look at other things (genetic traits of that ethnic group) like freckles, moles, Stork bites, Mongolion spots.
I have a friend that adopted twin babies. The mother was white, but the mother didn't know which man was the father. The father was black, and the babies don't look like they have any white in them at all.