This is a perceived circular argument due to the often misconstrued notions that Scripture is separate from Tradition. The mistake is presumed in this question:
the Holy Tradition that the Orthodox consider to be the second source of Holy Revelation and coequal with Holy Scripture
In fact, there is no circularity because Scripture is a subset of Tradition, and Tradition is "faith once delivered to the saints", i.e. the Church. We do not need to look for the authority of the Church in the scripture in order to justify the authority, -- although of course, if we were to look we would find it. Passages such as Matthew 18:17-18 to do not constitute a basis of the authority of the Church, but rather an evidence of that authority.
I'll be back Monday to respond to all posts to me, at this point I only have time to correct this possible misconception of what the Orthodox and the Catholic Churches teach (with one voice on this subject and on most other subjects).