The project is not terribly complicated.
You can download it here: http://sight-creations.com/fxexchange/SplitScreenSimple.zip
-------------- crash course in creating OSC controls for FCPX --------------
Start with a Line through the center of the canvas (leave some overhang)
In Properties > Tranform, reset the position to 0,0,0
In Shape > Outline, publish the Width and Brush Color (right click on each parameter to select Publish)
To make the OSC: Create a new group and set the Group to 2D and Fixed Resolution (very important!!)
To the Group, add a Poke (Filters > Distortion) filter and check the Publish OSC checkbox [You can use any filter with an OSC in Motion as a control in FCPX and link various parameters from the filter to other aspects in your project — but Poke is the most widely used.]
With the Group or Poke filter selected go to > Properties > Timing and set the Duration to the length of the project (the default is 1 frame - if you leave this at 1 frame, you will never see the OSC control in FCPX.) You can alternately move the playhead to the end of the project and with either the group or filter selected, type 'o' to set an out point.
Go back to the Shape > Properties > Position > X and right click > Add Parameter Behavior > Link
To the Source well, add the OSC Group
For the Source Parameter : Compatible Parameters click the dropdown and select Filters > Poke > Center > X
In the Behaviors > Link inspector, go down to the bottom and for the X offset, set it to -0.5 (very important)
In the Filters > Poke inspector: publish the Center (you must publish these parameters in order to be able to keyframe the position in FCPX -- the Line's position X parameter will not be keyframable linked to another object.)
**very important stuff**
Fixed Resolution: this forces the "reference rectangle" of the filter control to be *iimited* to the canvas dimensions. If this is not set, a slight drag on the control will send the object under link to "astronomical" distances.
X offset: The canvas is measured from a 0,0 CENTER with the left side at -0.5 and the right side at 0.5 (a "distance" or "multiplier" of 1.0) [same for top to bottom]. Everything in Motion is measured by this dimensional measurement times the canvas measurement (this makes all projects in Motion "scalable" to different resolutions.) Therefore, to realign the center of the object linked to the center of the OSC control, you subtract half the distance of the screen (0.5) from the offset. [In a 1920x1080 or a 1280x720, or a 4:3 resolution, -0.5 will always be aligned to center screen... hope that makes sense.]
HTH