It's not a question of co-existence for the two apps, it's a question of Apple's limitations on the "Open In..." function.
You can only see 10 apps in the "Open In..." list. So, if loading Adobe Reader is the 11th app that can read PDFs, it may very well push off an older app that was already in the list. There is no controlling this process - nor does anyone really understand how it works.
The only thing I know to do is to delete some [in this example] PDF-reading apps until the number gets below 10. Then, you can see both Adobe Reader and GoodReader when looking at "Open In..."
Hope this reassures - though not necessarily help. But, what can you do?
Marilyn
P.S. And you've further solidified my opinion that sometimes ... the reviewers in the App Store have no clue... !!