thats because you did not copy in binary mode
have a jpeg picture named 1.jpg ready
have a rar file of anything name 2.rar ready
create a batch file with this command
have all 3 files on the desktop

Code:
copy /b 1.jpg+2.rar 3.jpg
run script
it will create 3.jpg which has combined size of 1.jpg and 2.rar.
this 3.jpg can be opened to reveal picture of 1.jpg, and when renamed to 3.rar, it can be opened to show files which 2.rar contains.

i have some theories, it could be that windows picture and fax viewer reads the 3.jpg file in an order such that when it reads the header data of a picture data, it will display the picture, when win rar is used to open 3.rar, it will read the initial data containing the picture file but since it does not understands it, it skips pass that chunk of data and when it reads the header data of a rar file (something it was made to do) it opens it up as a rar file. this is because binary copy command literally copies the data of the second file after the first without mixing the data up. another interesting thing i noticed is that when this "hybrid" zip is opened with win rar, everything is fine, but when opened with windows native zip handler, "compressed (zipped) folder" it errors that file is corrupted.

can anyone tell me if my theory is right since i am not an expert in the area of how a program executes a file.

thanks again, all your help is appreciated.