When you do a foreach between two ATOM feeds, you get a "join". However, you also get duplicate "title", "id", ... etc. So you need to do a transform after this, right? To get rid of one set. So you pick a transform, and as a shortcut, pick one set of title, id etc from the left and copy it over to the output tree. And you create a new content element, and copy the two values (outer and inner) into it. Feeling good, (the output tree looks sensible) are you? But wait, you get to preview, and you find two "title", two "id"... what is going on? Quite simply, in transform, copying from input to output copies by name, not position. So even though you thought you had copied one over, by copying "title", you told transform to copy "title" over whenever it occurs at run time. The clean look of the output tree is misleading, at run time, the transform operator will copy title over twice.
So what to do? There are two options...
1. Either create new attributes on the output for title, id etc. and carefully populate them with values from the input tree (by specifying functions or text values or some such). Slightly painful, but works.
2. For the inner of the foreach (or outer, whichever you prefer), in Advanced tab, make it an XML as opposed to ATOM. That prevents all the unnecessary title, id etc. being created for each entry and being passed on to the foreach operator. So now simple copy of the outer's title, id etc. to the right works. Cute, eh?