If you want to use two types of film just buy a second camera.
No, it is not a crazy idea. The cost of film and processing (especially if you have to do mail order processing) will far exceed the price of an extra camera by the time you have used up five to ten rolls of film.
I thought I'd clarify a bit too since you dont say how old you are. I'm young (28), but I did grow up through most of my youth with my parents owning film cameras. If you are younger you may not have ever experienced it. The first time I learned how film works on a very basic level was when I was a kid and opened the back of the family camera. Needless to say, I got yelled at because I ruined most of the roll.
Sorry if this is elementary, but the reason you can't remove the film before its finished is that light exposes film. When exposed in the camera, you are setting very specific parameters so an image will show up on the film. If you just open up the film to daylight, it will essentially make your film useless. On 120 film, there is paper at the beginning and end of the roll that protect the film from being exposed when loading and unloading film.
On 120 film, there is paper at the beginning and end of the roll that protect the film from being exposed when loading and unloading film.
Just to clarify, the paper runs the whole length of the 120 roll, but on 220 film (same as 120 just twice the film length) the paper is only beginning and end of the roll to keep the roll diameter small enough.
Your Diana isnt designed for 220, which is fine because their is so little available and it costs 3x as much as 120.