If you have the parents of each set of siblings, or just want to add them as place holders until you do, that is enough to indicate the relationship. But maybe you're asking how to make that show up with the boxes and lines of the family tree. And there is no way to make both sets of parents appear on the tree at the same time in Family Echo, and I haven't seen any other family tree software that does that automatically either.
Family Tree Maker will let you move boxes and lines around when you are ready to print things out, but you'll have to think about how you want those boxes and lines to look. Here are two examples from RelatedHowAgain.wordpress.com that you could use as examples. The first one keeps the depth of generations information intact for the parents. The second one keeps the depth relationship intact for the siblings.
