Where a model has Attacks at two or more Initiative values, such as a knight or other cavalry models, resolve each set of Attacks in the relevant order.
If the model is slain before it can finish striking all of its blows (because one set of Attacks is at a lower Initiative than the enemy) then obviously these are lost, just as a model with a single profile would lose all of its Attacks if it were slain before striking blows.
Similarly, on rare occasions, a model with a split profile might be unable to strike some of its blows because all eligible enemies have been slain. Take heart from the fact that such situations are normally caused by your warriors having killed a great deal of the foe.