This can be done in the VSE with the speed control effect:

1. Select the video strip you want to slow down
2. Add a speed effect strip (⇧ ShiftA> Effect Strip > Speed Control)
3. Extend the original strip to twice its length

This can be done in the VSE with the speed control effect:

1. Select the video strip you want to slow down
2. Add a speed effect strip (⇧ ShiftA> Effect Strip > Speed Control)
3. Extend the original strip to twice its length