I have two separate library files with a function of same Name and Passing parameter. How can I use these two library files in a single project? Or how to change the function name in one library?
You normally can't unless you have access to the source code so you can recompile one of the libraries after having changing the name.
This is one of the big reasons why C++ introduced namespaces.
Maybe, just maybe, the C51 linker has some support for name aliases, but you'll have to wait for someone who uses C51 heavily to respond.
The chances of that happening must be really rather small - unless you are trying to use two libraries which both do essentially the same thing...