There are two images: a.jpg and b.jpg.
I just want to know how to join them into one image using ffmpeg.
How should I finish the ffmpeg -i a.jpg -i b.jpg command to get a c.jpg output image?
This is an example of what I am trying to achieve:
a.jpg

b.jpg

c.jpg

I think that tile is your ffmpeg command.
You can find more info on Superuser.
Try:
ffmpeg -i a.jpg -i b.jpg -filter_complex scale=120:-1,tile=2x1 output.jpg