How can someone have two CoC accounts on the same device?

Solution 1:

iPhone:

• You need a second Apple ID.

Android:

• You need a second GMail Account.

The other steps are similar in Android and iOS:

1. You have to link your progress in the CoC settings with either your Apple ID on iOS or your GMail-Account on Android.
2. After that you have to refresh the game data: This works efficiently by reinstalling the game.
3. When reinstalling you have to choose the correct GMail account on Android or link Game Center with the correct Apple ID in iOS depending on which account you want to play with.

Extensive guides: Multiple accounts with Android devices | Multiple accounts with iOS devices.

As you can see this way isn't very trivial and also probably not intended by the developer (otherwise it would be easier). That's why probably most people who really have more than one Clash of Clan accounts have either more than one device or play the second account in an emulator.

There are several android emulators for Windows and that is what your friends might have meant with the "additional software".

In google I found this topic, which you might find interesting: Best Android emulators for PC.