Well, one CGI invokation can only produce exactly one document. What are you actually trying to do? I get the feeling it is something that has to be done in a different way than you think.
What I'm trying to do is print two graphs on the same page like a bar graph and a pie graph, one next to the other but the cgi prints only the first graph and ignore the second one.
You need to pass your CGI a parameter to tell it which graph you want. It obviously can't know which part of the page it's being called for (in fact, it doesn't even know there is a link to it on a page). After all, if you type the CGI's URL in your browser address bar, how would it decide it what to do? Append something like ?type=bar to the URL and read up on CGI's param() function.