Now it's time put the four keys you mastered in Lesson 1 together with the four keys you just learned last lesson.
Typing with both hands is more fun and in fact easier and less fatiguing than typing with one hand alone. Plus, you'll gradually get more & more letters at your disposal and be able to type more actual words, rather than nonsensical strings of letters.
Keyboarding is of course a cumulative skill: everything you learn builds upon what you've learned before, so you mustn't forget what you've learned!
Remind yourself of the Principles for Effective Learning before starting....