- Open the first document ("File/Open")(preferably the one that will be on the left)
- Add the second document as another layer ("File/Open as/Layers")
- "Image/Canvas size": double the width: either set units to "%" and enter 200, or just add "*2" and press [enter] at the end of the width. Then click "Resize"
- Check "View/Snap to canvas edges"
- Use the "Move"
tool and drag the upper layer to the right. Due to the previous step it will self-align to the edges of the canvas as long are close enough.
Don't forget that to obtain a format other than XCF you have to use File/Export.