I get a lot of hits from search engines about merging two churches into one. Take the blog title The Merge and add a bit of discussion about church, theology, and Bible, and tada...you get placed on the search engine for people wanting to merge two churches together.
I don't know a lot about how to do it, but I do know one piece of information for those who are looking. I once worked for a denominational headquarters with a great research department. One of things that I noticed was that two churches of 50 people (or whatever number) became one church of 50 people after a merge. Not immediately, but over time. Very rarely did you get a church of 100.
I think that a pastor, leadership team, church style, whatever that fit the church of 50 does not automatically jump to being able to handle the 100 people. Therefore, it goes right back to being 50. Also the people who were at one church because they liked the pastor no longer like the new guy, and so they leave.
There are probably a million other reasons for the two churches not being able to combine into one, bigger church, but I saw the actuality of it happen over, and over, and over again for multiple sizes of church.
Addition: In an attempt to help you find what you are looking for...how to merge two churches...I am providing links to LifeChurch.tv's series on Merging Churches. I hope this helps you in your pursuit of furthering God's Kingdom in your area.
Should our Church Merge?
What to do during merger talks
The Public Merger Meeting
Why I Believe Partnerships are Important
Let me know if any of these help. Please feel free to leave a comment here to help others who are trying to merge their church with another.