You can join several input movies into one large movie with the desired output format and size :
Select "File" mode (default selection), and click "Browse" to select the first movie located on your computer.
Check that "Input format" is correct, and select the desired "Output format".
In the Options Box, select "Merge multiple input movies". Now you can click "Browse" and select the second movie located on your computer that will be joined to the first movie.
You can click "+" to add up to 9 input movies.
All movies will be joined and resized into 1 movie.
I have used this successfully before. But now when trying to merge multiple movie files into a single file, both files are obviously uploaded and processed, but the output file is nothing but the first upload file.