I believe you're talking about converting two mono items to a single stereo item. That's a direct edit on an audio source that Reaper, as a non-destructive application, doesn't do unless you render the items and insert the render into the project.
If you want one mono item on the right, one on the left, put them both (ie implode to one track as Fabian describes) on one track in Free Item Positioning mode (note the little tab on the bottom of each item), and use 'Item Properties to pan them left and right. For tandem editing fade-wise, etc, group them.
You can have them on separate tracks sent to the L/R channels of a parent track otherwise.