If an effect would simultaneously destroy Knight Exemplar and another Knight creature you control, only Knight Exemplar is destroyed.
If another Knight creature you control is dealt lethal damage, the creature isn't destroyed, but the damage remains marked on it. If, at some point later in that turn, you no longer control Knight Exemplar or it loses its abilities, the other Knight creature will lose indestructible and will be destroyed.
If you control two Knight Exemplars, each one causes the other to get +1/+1 and have indestructible.
Lethal damage, damage from a source with deathtouch, and effects that say "destroy" won't cause a creature with indestructible to be put into the graveyard. However, a creature with indestructible can be put into the graveyard for a number of reasons. The most likely reasons are if it's sacrificed, if it's legendary and another legendary creature with the same name is controlled by the same player, or if its toughness is 0 or less.