Stiching 2 Geotiff

I flew 2 missions right next to each other. How do I upload the 2 files to have them stitched together?

On the old interface there is a big blue plus sign at the bottom.

I know how to upload after the flight. I flew 2 missions and uploaded them separately on different days. I have the 2 separate Geotiff maps. Now i need to stitch them into one map.

Ok, thought you the images from two flights. Combine finished orthomosaic in QGIS.