Only use for 1 mouse to control 2 screens i can think of is if both screens are running same say video game, and your using multiple computers all controlled by the same mouse to carry out the same mouse event on multiple systems at the same time. 
I guess after reading it wrong in the beginning, after correcting myself I was spot on in this statement other day.
If you want to automate video games its best to use a keyboard/mouse macro. Run through a loop of a routine recording your mouse and keystrokes in real time. Leave a known position and come back to that known home position when done. By doing this you can then have your player in the game do whatever the process is and you dont even need to be there to control it. I do this with some redundant time wasting features of games such as farming materials that have time delays aka cool downs. I use a software which wasnt free but it works really well called Jitbit Macro Recorder. I can press record and whatever i do with keyboard and mouse is recorded in real time. Then press stop when done. I can then compile this routine as an EXE and call upon it as a scheduled task. I broke down the processes into small redundant chunks and placed them into a batch file listing such as:
go_farm_wood1.exe
go_gather_gold1.exe
each time the batch file is called it runs through the farming of wood and then after thats done it goes and gathers the gold. If you gather items very soon to the intervals that is the wait time for them to be gathered which is a timer, you can make a lot of money and gather wood quickly as if you were there clicking when it was complete every say 5 minutes 24 hours a day, but you dont have to do that the computer does all the brain sore work so you can enjoy life and just play the fun parts of the game which is not the gathering part.
For this to work though the games cant have sniffers. There are games out there with memory sniffers that look for cheaters. If its a game that has memory sniffers then you risk permanent ban from the game if caught. I dont look at what i do as cheating, I look at it more as making better use of my time to play the fun part of the games design and the boring part of the design i leave up to the computer to do for me.
One game I play it was consuming way too much time 1.5 hours of each day was gathering stuff and going through a daily process after that for 4 accounts that I play on. I now can play the 10 or 15 minute fun part of the game and leave my macros up to doing it all unattended for the boring part. So I start my automation before bed and let my computer play the game for me when I sleep. When i wake in the morning everything has been gathered and I can check on stuff and play the fun part.
Thinking you will be better off using automation macros for what you want to do... however... if the game is dynamic, that is targets are not always in the same location at timed intervals, then it wont work for you unless you have a bot with object recognition running that can target stuff and assume preprogrammed instructions of what to do when specific objects are detected etc. I myself havent bothered with dynamic automation. Everything i do is redundant always the same automation so every time its run the objects to interact with are at the specific coordinates.
That Y splitter with 2 laptops is VERY dangerous.... Each laptop has its own 5V DC power and you basically merged the USB port power between them. If they are balanced voltages no problem, but if one laptop is 1 or 2 tenths of a volt different than the other, then power from the one that is 5.1 VDC will be traveling to the laptop that is 4.9VDC and you might cook the USB ports as you back feed power into the 4.9VDC laptops USB port. The other issue you have is the serial communications cant just be teamed like that for both to detect the mouse and run with it.
The only hack I can think of would be to take 2 mice, and wire one mouse to the other with opto isolators for its X,Y and relays that trigger for the Left/Right mouse click so that each PC is interacting with the chip of each mouse, but by which the mouses information is teamed and isolated. *Its more work than its worth to pull this off. The other issue you will run into is that both accounts will need to have the game starting at exactly the same location for coordinates of the player and both have the same environment objects, so when clicking to pick up an object on one laptop for example, it picks up that same object on the other laptop running the same game. Any little offset will become a greater and greater offset in the game as you move further and further away from your starting position. I have lots of experience with the smallest offsets causing BIG Problems with automation!
