I am a big fan of playing games on a tablet, but having to set the tablet down to hold a controller has always been an annoyance. I have seen a few people out there make these split controllers before but there is very little documentation on how to do it. Not that I did an amazing job, I'll point out the many mistakes as I go, but here are a few steps to help out along the way. I did get some very good pointers from SpecializationIsOverrated. Very cool guy that makes some interesting stuff.

I wanted my controller to be universal in case I ever decided to use it on another tablet. This complicates things since making a split controller is actually the easier portion of this effort. Attaching it is where the imagination comes in.

Things you need:

Required

1) 2 wired 360 controllers. Get ones made post 2012 if possible. Makes wiring much easier.

2) Solder and a soldering iron.

3) Dremel or something to chop down the case of the controller.

4) Hot Glue helps to secure wiring and avoid damaging solder points.

5) Controller mount. I used a cheap headrest tablet mount but if you are savy, you can do something else. Gluing both sides of the controller to a tablet case could work if you don't mind the look.

Not Required

1) Sugru helps to seal holes and add some traction to the points that will actually touch the tablet sides.

2) Instamorph. I use this stuff to custom mold covers and make plastic parts align better. It is very useful stuff.

3) JB Weld. I use this to fully secure the controller to the extending mount. Any other adhesive should do it but I have a deep affection for JB weld sticks since they are very moldable and cure incredibly strong.

Note on that picture up there, I think I was holding the thing oddly since that right controller is not actually crooked. That just looks embarrassing.

Step 1: Breaking Down the Boards and Soldering

Easiest way to do this is to use two controller boards. One controller remains the host and retains the USB cable (the left half in my case) while the second controller exists only to remap some buttons (right half in my case). Using controllers made post 2012 helps here since the later model uses a common ground for the buttons. This reduces the number of necessary solder connections by half.

Even so, you will need to connect something close to 20 wires here. There are some guides out there that do a great job of mapping out all the test points. This saves you from having to solder to the buttons themselves. Saved me some headache since I accidently pulled the entire solder point for the right bumper off when I removed the mechanism. Try searching Slagcoin for 360 controller pcb wiring. That should explain things.
I prefer braided ribbon cable for this part. The solder points are not small so cable wrap is unnecessary. Cable wrap is also terribly flimsy and doesn't bend well. I would suggest you work to make these wires all the same size and to add heat shrink tubing before hand. I failed both of these suggestions and regretted that later. Also check that your wires will be long enough to actually fit a tablet between the two halves.

If you want to shorten that usb cable (and you do) now is the time to do that. Doing so after you close the controller is not pretty.

Test when you are finished here since it will be difficult to take everything apart to fix anything. Once you know it is working, use hot glue to hold down those wires. I completely cover the solder points just to make sure nothing moves. The hot glue also keeps you from damaging anything when you pull those wires, which you will do later.

Also, as soon as you power the right half of the controller, that guide button is going to start blinking and will never stop since it will never sync to anything. I got around this by simply covering the LED's, but you could probably just remove or short them without ruining the circuit.

Step 2: Cut Out the Controller Case

This is one of those "Measure twice, cut once" moments. I decided that I wanted a functioning home button on both sides in order to make a more symmetric controller. I then decided to chop the controllers exactly past that point.

Later on, I wished I had left the back plate on the controllers to protect the PCB board. This would have left a better structure to secure to the extending mount later on. In some cases, people have simply glued that back plate directly to the tablet and had a permanent controller attached.

My bad decision forced me to create a custom and ugly cover for the PCB board using instamorph.

I would test everything again here. It is tough to align those rubber pads on those buttons since you have to cut them down. Make sure everything is placed correctly and all the buttons have good travel before sealing up.

Step 3: Attach to the Mount

Lots of steps skipped here. I initially wanted to make an awesome extending mount out of wood that would slide and extend nicely. It was supposed to house the controller board and have a great enclosed look to it.

I then remembered that I have no idea how to do any of that and had little interest in spending hours in my very hot garage figuring that out so I found a cheap mount on Amazon to use. I ended up using one of the tablet mounts that is built to attach to the back of a headrest. Sadly, it was intended to attach to a tablet diagonally so I had to cut off and remake the grip ends with instamorph in order to get it to attach flat.

Holes were filled and gaps were smoothed with some more instamorph and the whole thing was secured with a JB weld stick. Amazing stuff that works like sticky tack then quickly hardens.

I also shortened the usb cable and did a terrible job of hiding it using some heat shrink tubing. Thinking back it would have been nicer to have the cable come out through the side of the controller and remain hidden under the tablet.

That rats nest of wires was also brought into line using some Xtreme tape. Silicone tape that works well in wrapping wires when you foolishly forgot to put heat shrink tubing on them earlier. It bends and flexes well and will protect the connections nicely.

Step 4: Paint and Admire

I am a better painter than a carpenter, which doesn't say much. I decided to not repaint everything since I didn't want to ruin the feel and texture of the parts that I would be holding. I used plastic wrap to cover the controller itself and leave only the instamorph and JB Weld sections exposed for painting.

Went though my garage and found that I was out of primer and only had some glossy black. Did the job but I would have preferred a few sanded coats to even everything out and a more matte finish.

Honestly, this was good enough for me and it is functional which was my bottom line requirement here. I would rather spend time playing with it now rather than perfecting it.

Either way, good luck and I hope this was helpful.