I was wondering since it is not indicated in the 550's specs in Microsoft's site. Even the link you gave was not sure because it said that it is only 'apparent' for them to be dual sims.
I just thought of something. What if it is only dual sim, like how the 535 is? Someone made a comment about the other sim being at the top of the phone for the 950, and it looks like that can be the case here too.