Bojan is right, what you are trying to do by stacking is increase the signal to noise ratio. By only having 2 light frames and no dark frames, you havent really accomplished that in any major way.
The Deep Sky Stacker website has a good explaination on the why and how
http://deepskystacker.free.fr/english/theory.htm
As it says in the opening lines.
Why combine?
The answer is simple: only to increase the Signal to Noise Ratio (SNR).
Is the resulting image more luminous? No.
Is the resulting image more colorful? No.
The goal of combing many images into one is only to increase the SNR. The resulting images are neither more luminous or more colorful but they contain much less noise which will let you stretch the histogram a lot more which will give you more freedom to bring back colors and details.