What is your question exactly? Are you trying to take the two together? Are you using the Ketamine to aid in a more comfortable comedown experience?
Ketamine is a sedative, essentially, in that when you take too much of it, you're going to lose consciousness one way or another. It is however not dangerous in the same way that say, an Opioid would be. Ketamine does not significantly depress the respiratory system in the same way. This is why Ketamine has become popular in emergency/battlefield medicine as it is less likely than an Opioid to kill, though it can also disrupt or dissociate a person from their pain response.
So, you are not exactly walking through a minefield here. I would just advise you to know what you're doing with both of these drugs prior to using them together at a club or event or something. Don't just take a tab of MDMA and then start dipping your finger into a baggie of Ketamine. You need to know what you're capable of handling. Although Ketamine will likely not kill you, being unconscious in a public place among strangers is a dangerous situation of a different variety. It can lead to robbery, violence or encounters with law enforcement that can ruin your night or even your entire life if you really push it.
Take the MDMA, enjoy it. Use very small amounts of Ketamine until you know exactly what you can tolerate and how the drug(s) effect you. If you have friends who know, I would ask them to help you out. If you're just going out alone, I reiterate, you really should be familiar with both of these substances before going out into the world with them.