Here's an example of some (partial) success at reuniting a broken group. The group to Civil War Col. A. G. Tassin was broken up in a Stack's sale in 2005 where I bought the French Legion of Honor. A few years later I was able to acquire the Loyal Legion badge as well. Just recently the GAR medal in the center showed up on eBay. (Someone lost a bit of money on that medal. It sold in Stacks for $550 but I got it on eBay for only $258. Go figure...)
Unfortunately there's also a beautiful Army of the Cumberland badge to Tassin that's still MIA. It sold for $3200 in 2005 so even if it eventually surfaces I probably wont go after it. In the mean time I'm happy with what I have. It is possible to reunite groups with a bit of patience and luck.