Why does John not kill these two?

There's no specific reason given in the film, so this is how I understood it.

It's clear that the assassins appear to be enjoying their fight with John, and there are definitely a couple of points where they could have killed him, but don't do so. Their boss is a huge fan of John's work/skills, and he wanted the honour of fighting (and killing) John himself.

However, even with the two of them, John still manages to overcome them. John respects the abilities of the assassins and knows that they had those opportunities, so when he has the chance to return the favour by not killing them, he does so.

With the exception of his explicit target, John Wick has typically avoided killing in cold blood. Everyone that John kills was also trying to kill him. Though he stops if he is capable of incapacitating them in some other way.

For example, in John Wick 2, he stabs Cassian in the heart but (seemingly) in a non-fatal way. He leaves Cassian alive "as a professional courtesy" and exits the train. Cassian is no longer a threat at that time so he leaves.

Likewise, he did pretty much the same thing to Ares. He stopped as soon as she was no longer able to fight. He seemingly left her to die after a fatal stabbing, but we don't see her die so it leaves open the possibility of her surviving. Either way, she was no longer a threat, so he left.

In the case of the two assassins in John Wick 3, he incapacitates both of them when they fall through the ceiling. It is clear that neither of them can move , so he leaves them to deal with his current threat which is Zero.

Whether this behavior is part of his own moral code or another rule in the complicated world of assassins remains to be seen.