The polyphony and polyrhythms resulting from interlocking parts really come alive when two mbiras are played together. The different paths the two mbiras take—referred to as kushaura ("to lead" or "to start") and kutsinhira ("to intertwine with")—are in an interlocking relationship with one another, one player sounding in the silence of the other, thus forming figures of intricacy and variety exceeding the movements of the fingers alone.
The kushaura and kutsinhira are usually referred to as "parts" ("You play the kushaura part...") but it is more useful to think of them as roles, as expressed in verbs such as "to lead the piece, to take the solo part" and "to exchange parts". Speaking of them as parts is useful — we can generalize that the kushaura usually contains the melodic core of a piece and does not emphasize the mbira's bass notes, while the kutsinhira often contributes rhythmic contrast and does emphasize bass lines. And it is also true that most mbira pieces have variations that are commonly played as kushaura and others that interlock when played a step behind.
But the primary importance of the two roles is to create mbira's signature interlocking of the two parts, with the two mbiras maintaining different rhythmic positions. Cosmas Magaya has emphasized that first and foremost one mbira's right hand notes are always one pulse behind the other's, maintaining the "hocketing" or interlocking effect when playing high-tone melodies. (The common generalization that the kushaura's right hand that is ahead of the kutsinhira's has significant exceptions in songs such as Nhema musasa, Mahororo, Nhema musasa yepasi, and some versions of Bangidza.)
Most mbira pieces do have variations identified by mbira players as kushaura and kutsinhira parts. Here is one kushaura for Nyamaropa:
And here is one kutsinhira for Nyamaropa
Here are the kushaura and kutsinhira for Nyamaropa being played together.
Mbira songs have numerous kushaura and kutsinhira parts. Although you can theoretically play any kushaura variation for a piece with any kutsinhira variation, in practice experience and listening lead one to match particular kushaura and kutsinhira parts.
Several mbira pieces have a kushaura part and a high kutsinhira part (called in Shona kwepamusoro) that can be played behind the kushaura. This high part can, however, also be played as kushaura, with other kutsinhira parts taking the kutsinhira role. Two "big" songs in the mbira repertory have this high part that can be played as kushaura or kutsinhira—Taireva and Mukatiende.
Here are three parts to Taireva:
You may hear these songs played with the traditional kushaura parts, parts that very much uniquely identify these songs. But you may also hear them without these signature kushaura parts but rather with the high kutsinhira as kushaura. Here is Taireva with and without its signature kushaura:
Not all mbira pieces have separate kushaura and kutsinhira parts. When playing songs such as Kuzanga, Nyamamusango and Shumba the kutsinhira player plays (basically) the same part as the kushaura but one step behind. Here is Nyamamusango, first with just one player and then with a second playing the same part interlocking one step behind.
Because kushaura and kutsinhira are roles or places in the rhythmic pattern, one can theoretically play any two parts together, as long as the two parts interlock properly and follow the same harmonic progression. So one could, for example, play two kushaura variations of an mbira piece together, with one "behind" the other. Of course it takes a lot of experience with the music to know when this effect results in good mbira music.