There are no side altars in the nave of the Church - only in the sanctuary are there side altars, and they are used for the preparation of the Divine Liturgy. When more than one priest wishes to celebrate the Divine Mysteries, he joins the principal celebrant and CON-celebrates. This has been the custom of the Eastern Church since time immemorial. The iconostasis is a partition separating the altar area from the body of the Church. Three sets of doors opening into the altar area, and are used frequently during the services.
Some icon screens are plain and simple in design, others are highly ornate.
The doors of the icon screen will usually be closed at this time, unless it is the period between Easter and the Ascension. But you can hear quite well all that's happening in the sanctuary area. It is at this time that the celebrant, standing at a side altar behind the iconostasis, prepares the bread and wine that will be used in this liturgy. When he has completed his preparatory prayers, the central doors of the iconostasis are opened, revealing the altar. When the steeple bells ring, the congregation stands.