Gav your question has two issues.
1) technique to combine images from different cameras and/or sessions of photography
2) programs that can do it
Combine images from differente cameras or sessions is an exaustive task. Some times you need much time and effort.
Problems with alignments, rotation, scale, sharpness of images, background glow and others call for many adjustments and graphic process.
Alignments and rotation can cause others issues like natural distortions of periphery of the photo with different areas of the target between photos. Some times you can align the left side of two photos but the right side will be out.
I did something with Photoshop CS3: adjustments of histogram, alignments. Mainly working with layers ressource.
DSS can do someting, also. You need work with group tabs. It accepts different ISO or cameras types with this ressource.
Therefore it is possible and there are some beautiful works with combining. But, warning, it is not an easy task.
