You are correct in saying that elders are always found in the plural when it comes to their service in the church.  Let’s look at a few examples:

Acts 20:17 and 28, “And from Miletus he sent to Ephesus, and called the elders of the church.  Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.”  In the early church, a local church was identified by the city or region in which it was located.  Here, the Apostle Paul is addressing the ‘elders’ of the church of Ephesus.  He points out that it was the Holy Ghost that had made them elders and their function was to ‘feed the church of God’ which was in Ephesus.

Acts 14:21-23, “And when they had preached the gospel to that city, and had taught many, they returned again to Lystra, and to Iconium, and Antioch, Confirming the souls of the disciples, and exhorting them to continue in the faith, and that we must through much tribulation enter into the kingdom of God. And when they had ordained them elders in every church, and had prayed with fasting, they commended them to the Lord, on whom they believed.”  ‘They’ in verse 21 refers to Paul and Barnabas.  In their return trip to the cities mentioned, they ordained “elders in every church.”

Titus 1:5, “For this cause left I thee in Crete, that thou shouldest set in order the things that are wanting, and ordain elders in every city, as I had appointed thee.”

Let’s consider the words of the Apostle Peter in 1 Peter 5:1-3, “The elders which are among you I exhort, who am also an elder, and a witness of the sufferings of Christ, and also a partaker of the glory that shall be revealed: Feed the flock of God which is among you, taking the oversight thereof, not by constraint, but willingly; not for filthy lucre, but of a ready mind; Neither as being lords over God’s heritage, but being ensamples to the flock.”  Peter, who identified himself as an elder, here addresses the local elders.  Even as Paul exhorted the elders in Ephesus to “feed the church of God”, Peter here exhorts these elders to “feed the flock of God which is among you.”  This suggests that the role of the elder is to a local church or assembly.  As you mentioned, and as Peter states, the role of the elder in the local church is ALWAYS mentioned in the plural form.  There was ALWAYS more than one elder in the local churches or assemblies.  How were these godly, aged men to function in the local church?  It was by taking the ‘oversight’.  Note that it is not the ‘overrule’, but ‘oversight’.  These men were to be examples to the flock among them.  This is probably the most difficult, but most effective way to lead…to lead by example.

There are qualifications for one to function as an elder or bishop.  We find those in 1 Timothy 3:1-7 and Titus 1:5-9.  If one desires to function as an elder in the local church, he MUST meet the qualifications given.  For one to truly be an effective elder, his appointment must be by the Holy Ghost as we saw in Acts 20:28.  It is probably to prevent the possibility of one taking control of the local church that instruction is always to the ‘elders.’  So, we see the pattern of scripture is that the local church or assembly is cared for by the loving, patient, experienced hands of two or more elders.  (360.2)