How to start two Ubuntu applications in one go?

Lets suppose that there are two binary files, e.g. XXX and YYY, at destination /usr/bin/. I want to start them at once, always! In other words, when I type the name of either of those in unity, I want to make both of them start. How could I change the corresponding files at destination /home/username/.local/applications/ to do so? I tried this /usr/bin/XXX;/usr/bin/YYY; on terminal but it doesn't seem to work.

As I understand you need to edit your .desktop file. Add this line next to Exec:

sh -c "/usr/bin/XXX & /usr/bin/YYY"

The -c flag tells to the shell that it has to read the commands from string, instead of from the standard input.

If you use only /usr/bin/XXX & /usr/bin/YYY it won't work. Just to add the reason why & doesn't work in a launcher, it's because & is a feature of the shell, and the launcher isn't a shell, its a much simpler environment for running commands.

/usr/bin/app_first & 
/usr/bin/app_second

Using '&' you are sending process into background.

You should use

/usr/bin/XXX && /usr/bin/YYY;

Note the use of && (no need to use & to run something in the background).