Plagiarism in journals
An interesting one, although I wouldn't jump straight to plagiarism. 

Sometimes, if the authors are all in the same research group/university/project (this could easily be across countries) they'll use bits of colleagues work with less rigorous citing than they would normally. This is especially the case if the first paper was mostly just a positioning or introductory piece.

It's not a great thing to do, but I've seen it happen on a number of occasions. However, I'd definitely get in contact with the journal of the second paper, and then if you don't hear anything constructive back then get in contact with the journal of the first paper. Definitely don't contact the authors directly unless you don't hear anything from either journal - if that's the case, get in contact with the authors of the first paper.