I'm looking for a way to separate the X and Y positions of Optical Flare using expressions.

It is almost always way more difficult to shape a motion path if you separate dimensions. Separating dimensions means no bezier handles on the Motion Path, no way to reshape the motion path in the Comp panel, and almost every other operation that gives you control over the actual path of your moving layer becomes more complex. 

If you have to separate dimensions, putting them back together using Dan's expression is the only way to do that. 

Maybe like this:

x = thisComp.layer("L1 Ctrl").transform.xPosition;
y = thisComp.layer("L1 Ctrl").transform.yPosition;
[x,y]