Animal behavior can be interesting to watch. Remember it is very easy to get discus to foster raise. When their hormones/instincts kick in they are very strong. The other fish is simply "wanting" to raise fry. The parents are not running it off because it does not appear to be a threat. On a slightly different note I have seen times when 2 females and 1 male would spawn together and raise the fry. I have also seen 2 males try to spawn together.
-john