Five : The Order for Practicing Shamatha and Vipashyana
Generally, first you practice shamatha and after that you practice vipashyana. That is according to the Theravada tradition. But in the Mahayana, Vajrayana or Mahamudra tradition, it is not always the case. Sometimes they can be practiced simultaneously depending upon the individual practitioner. Your teacher should decide what is best for you, as long as the teacher is qualified in meditation.
One result of accomplishing shamatha is to know the minds of other beings. An accomplished teacher uses this ability to see what is best for their students. The method for doing this is the same as remembering the past, but here the teacher concentrates on the minds of others instead of on themselves. This is of course easy to say, but not so easy to do.
The normal order is to practice shamatha first then vipashyana, and it is best to do it this way.
Six : The Levels of Shamatha and Vipashyana
How to unite shamatha and vipashyana? It is possible to practice vipashyana without shamatha, but it is not advisable. You can go to a teacher and receive vipashyana instructions, and use your confidence and intelligence to accomplish the practice. Even though you can have direct experience of the nature of things, this experience will not become stable without first accomplishing shamatha. This is also true for practicing vipashyana without a shamatha practice that has become natural. It is comparable to a candle in the wind; although it provides light, it is very unstable. Similarly, you can have a direct experience through vipashyana, but without shamatha it remains unstable.
On the other hand, if you practice only shamatha without ever practicing vipashyana, you cannot become liberated from samsara. This was explained before, in the obstacles to meditation. Accomplish shamatha without practicing vipashyana carries the risk of being reborn in long-lasting meditation states, which are still in the domain of ego. In the final achievement of shamatha, mind is in a profound rest. It is deeply relaxed, beyond what we can now imagine. But ignorance, the root of illusion, has not yet been removed. That explains the necessity for practicing both shamatha and vipashyana.
How can we unite them into one practice? This is not something we can accomplish yet. You can work with them in certain ways, but it is only when you have achieved the highest level of shamatha, that you can unite them completely. The ninth level is to rest the mind in equanimity. At that point, vipashyana develops naturally, and the two practices become one.