I enjoy MDMA and then topping up with the ket but thats simply down to preference.
Dose as normal for your MDMA experience and then start taking small amounts of ket until you start to get where you want - after a while you will know how much to dose at the start to get you wobbly without vanishing down that hole (unless thats your plan).
With ket when you dont know what your doing - do small amounts/bumps, dont do lines as you would coke/speed etc
To answer your questiong - MDMA and Ket, is an amazing combo