You can Join two falt files using Joiner and as well as Looup. As far as performance is considered Joining using look up would give you better performance.
The only way to join two flat file is joiner transformation if the structure of the two flat file is same and the datatypes also matches we can use union transformation but it wont join the flat files it will merge the flat flies. In informatica the union transformation will act as a unionall function.
