I will have to disagree highly with Ben (SolaScriptura) and Kevin (Kevin). They said something similar and I will just quote a couple things they said.
Kevin said, "try several different things if possible, i.e. 1st Sunday prayer meeting, 2nd Sunday teach the WSC, 3rd etc."
Ben said, "If a prayer service and a Bible Study fits better than another worship service, then try that."
If you are going to have an evening service in order to keep the Lord's Day holy and have the members do so as well, then why have a prayer service or why have a bible study when you can hear the preaching of the Word of God?
You do all of those things (even teaching the Westminster Standards) via a worship service where the Word is Preached faithfully.
As Presbyterians, the WLC should be sufficient to prove this case. It states,
Q. 153. What doth God require of us, that we may escape his wrath and curse due to us by reason of the transgression of the law?
A. That we may escape the wrath and curse of God due to us by reason of the transgression of the law, he requireth of us repentance toward God, and faith toward our Lord Jesus Christ, and the diligent use of the outward means whereby Christ communicates to us the benefits of his mediation.
Q. 154. What are the outward means whereby Christ communicates to us the benefits of his mediation?
A. The outward and ordinary means whereby Christ communicates to his church the benefits of his mediation, are all his ordinances; especially the Word, sacraments, and prayer; all which are made effectual to the elect for their salvation.
Q. 155. How is the Word made effectual to salvation?
A. The Spirit of God maketh the reading, but especially the preaching of the Word, an effectual means of enlightening, convincing, and humbling sinners; of driving them out of themselves, and drawing them unto Christ; of conforming them to his image, and subduing them to his will; of strengthening them against temptations and corruptions; of building them up in grace, and establishing their hearts in holiness and comfort through faith unto salvation.
If you want the best for your congregation, you will have a time where you preach the Word to them. If you have the opportunity to do it twice, take it! It sounds like the Session wants this at your church. Take that opportunity. If I wasn't lazy, I would suggest we do it even more on the Lord's Day. Center ourselves around God's Word and let us hear Jesus speak to us through His messenger and by His Spirit.
If you have a choice between prayer only, bible study, or Preaching the Word which occurs in a worship service where there is prayer and teaching of the bible, then go for preaching the Word. It is especially that which God uses for effecting salvation and sanctification.
So you are on the right track at looking at what a worship service should look like in the evening. In the south, regularly the congregation desires the evening service, so it isn't out of the ordinary for them.
All that you need in a worship service is Read/Preached Scripture, Prayer, Read/Preached Scripture, Singing, Read/Preached Scripture, Read/Preached Scripture, Read/Preached Scripture.