Tim, you make an excellent point. I have largely tried to avoid having simultaneous narrators who are intimately interacting with each other, because I think that muddies the water, but it also does give you just one side of the story. On the other hand, it lets you imagine the other side, and play devil's advocate, and that can also be fun.
I don't think that Wally and Clara come off as evil or mean. Will's comment about them comes to mind, when he said that he didn't like them, but he still thought they were nice people.
And I don't think you have to imagine how Brad would have reacted; all you have to do is re-read Paternity. :-)