vstack

The following command will create a video from two input files and place them side by side using the hstack filter.

ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack output.mp4;

More information on the hstack filter:

11.104 hstack
Stack input videos horizontally.
All streams must be of same pixel format and of same height.
Note that this filter is faster than using overlay and pad filter to create same output.
The filter accepts the following option:
inputs
Set number of input streams. Default is 2.
shortest
If set to 1, force the output to terminate when the shortest input terminates. Default value is 0

You can use the vstack filter in case you want to place the one video above the other.