When performing a chain of heavy attacks on a player, each stun applies the instant the melee weapon hits, keeping them in a 'stunlock' whilst the attacks are performed, when you attempt to stunlock infected in the same way they are stunned by the first hit but the second hit will not stun them again while they are in the stun animation from the initial hit, they will instead remain in the original animation until it finishes and then return to an un-stunned state immediately with the second heavy attack having had no stun effect on them. Please see the examples in the video below:
By contrast, here is a player being hit into a heavy attack combo stunlock: