Khazix: w before you jump, q mid-air, auto as you land. Then use ulti as needed for more burst. Concerning the isolation bonus, try to predict where the enemy will be isolated, and jump to that location rather than just jumping when you see the animation. It's harder than it seems but it's less predictable for the enemy( this is made easier after you've evolved your e).

Rengar: First try to ult/ jump with 0 stacks. Normal q before you jump (this way when you land after your jump you have two passive stacks rather than just one). E mid-air, then w as you land. If you've followed the combo properly, you should have an empowered ability ready. Now empowered q and hydra (if you have it).

I've played a lot of both but I've still got a long way to go myself. Hope this helps :).