You can't expect to slam the bits for two different images together and have a browser see the resulting bits as two images. The image formats just don't work that way. You can, as Beatnik suggests above, use GD to construct a (single) larger image out of multiple smaller images.

An alternative is to invoke your CGI twice from within an HTML wrapper, producing a different graph for each invocation. Unless both graphs involve a complicated derived dataset, I would think that producing one graph per CGI invocation would be simpler.