What if we send humans through the double slit? Lets say that we have a universe that has just 3 things inside, me in a specially designed suit capable of keeping me alive while near absolute 0 as to enlarge my de broglie wave-length, double slit setup and a wall representing boundary condition. There is nothing else inside the universe, nothing that would cause decoherence.
Would I pass through both slits (the slit system) as some sort of "jelly" and interfere with myself and create interference pattern at the wall, or would I just pass through 1 slit and still create an interference pattern?
 A: The slit apparatus is just a measurement tool for entanglement. So your question boils down to: "Is it possible to entangle a macroscopic living entity?" and the answer is....yes.
It has already been done with tardigrade, a few mm long water bug who is able to survive to almost anything you throw at him, from nuke to deep freeze.

Some researchers claimed in this paper to have entangled one of these fellows with a superconducting quantum bit.
There is another paper describing entanglement with bacteria. So it seems we are on our way to macroscopic quantum mechanics.
Will it be possible to do it with a living human? No, as long as we don't know how to revive us from a near zero temperature freeze.
Physicists are still investigating the transition of QM to macroscopic Physics. Bose Einstein condensates are the state of the art artificial many bodies quantum edifice. Double slit experiments have been performed with them, but they are still short of the number of atoms in a bacteria, let alone a human body.
A: Only one of you will not produce an interference pattern.  In the usual (non-macroscopic) double slit experiment the pattern only appears when you have a stream of particles so your simple universe (which is actually composed of an enormous number of atoms) won't give any useful result.
Each "you" will pass through a slit (or splat on the wall around them).

There is nothing else inside the universe, nothing that would cause decoherence.

This is just wrong because there are so many atoms involved in a human body not to mention the wall with the slits that decoherence is guaranteed to be an issue.  You just cannot ignore decoherence as you want to.  It will smoother any potential interference pattern to the point that it's undetectable.  Even if you had a stream of "you particles" to try and detect a pattern with, the result would be a pattern with two regions where you went through one slit or the other and no interference would be likely detectable.
