What do you mean exactly for "include directory path"? If you have to specify more than one directory in an environment variable, the common way is to separate items with a colon (as for the environment variable PATH). However it depends from which application will try to use that variable and if it can interpret multiple items this way. On the other hand, if you have to specify path to includes when compiling a source code, you can specify multiple -I options, e.g.
gcc -I/usr/include -I/my/include/dir source_code.c