There are too many ways to join FLV files. Here I just list three of them that I used:
Method one:
HandBrake. It can join FLV files effortlessly. The only strawback is this tool is a little complicate.
Method two:
An easy-to-use yet professional
FLV Joiner. But it is not free.
Method three:
Combine FLV files with ffmpeg at:
https://gist.github.com/halbtuerke/631