The answer to the question, "Why only two parts to this music?" has, ironically, a three part answer:
1) Practical: This music is ideal for mission parishes, older parishes with just one dedicated choir director (and a few singers) as well as for daily services. However, it is also apparent that many choirs no longer have trained singers. It is realistic to say that for male voices, very few men are real tenors and very few are real basses. Most male voices are baritone. Hence, it is actually rather unethical to force untrained singers to sing out of their tessitura (outside their given vocal range.) When basses do this, it will just sound silly. However, men who sing tenor without using proper techinque can permanently damage their voices. The Russian style of four part singing was developed in a time when singers were plentiful and often professional. This is no longer the case in some parishes (especially mission parishes.) This does not mean, however that four part singing should be done away with altogether. It obviously still has a place in the hearts of many of the faithful and should not be taken from them by force. However, it is important to look at one's own parish liturgical situation critically. We must ask: Are we forcing people in our choir into molds that are no longer appropriate and practical for our circumstances? In either case, we must proceed with great care and caution. Music in the Church is a delicate pastoral issue. The choir director must always be in dialogue with his or her priest about any changes in the musical repertoire of the parish. Choir directors must be sensitive and yet practical: What is going to work and what is going to produce an atmosphere of prayer and worship for my local worshipping community? The goal is not necessarily great sounding music (it is a great and necessary plus, albiet secondary) but rather prayer and communion with God. God is the end or rather the goal, and music is one of the means to that goal, not the other way around.
2) Textual: It seems that singing any more than two parts of music at one time can make it very difficult to really focus on the text (however, not always.) Oftentimes, the choir director and members are totally focused on making the four parts of the harmony blend and come together. The music in this case sounds nice, but this focus can make the worship feel superfical and too outwardly focused if not even showy. If the heart of a singer can not focus on the text, the text loses its power because it comes from the head and not the heart. Music that is sung from the heart is powerful and transformative. All Orthodox Chant is quintessentially text oriented: the music ideally illumines the text, highlights the text, and communicates the text. If this is not being done, either for the singers or for those who hear, what is the point? We lose our focus and goal if the sacred word does not lead us to REAL contact with the Eternal Word.
3) Ascetical: The ascetical dimension of all Church music is an ever present companion of all choirs and chanters. However, with Orthodox chant there should be an added dimension of austerity. Since music is a reflection of life, it is reasonable to state that since the Orthodox ethos is very ascetical, our music should also reflect this worldview: sobriety, restraint, cutting off the will in order to follow the traditions and prescriptions of the Church. Is it time to simplfy in order to reflect a deeper quality of Orthodox Life? It was once said that the return of the Byzantine style icons in the early mid 1900's would also neccesitate a change in its liturgical companion, Orthodox music. Are we ready for a change and what does this really mean? The answer to this can only come through prayer, individually and corporately.