The Pokefan in in Blackthorn told me Raikou and Entei were spotted on Route 40. I went there and eventually caught Entei.
But then Raikou would not appear even after several thousand encounters!
I kept double checking with the Pokefan but nothing changed.
It could be that I had very very skewed luck but I wonder if there's some bug around catching a roamer locking the other one out until they move on?