According to me this can happen for sure. In my personal case my full time job f.e. I'm a Project Manager for European Institutions and NATO. However I have a business of my own where I sell crochet creations (https://www.facebook.com/vanvie.be/) which I work on during my spare time. Both jobs have nothing to so with one another.
On the other hand there are thousands of examples of people doing one job for years after graduating but decide after decates it is not what they like to do and pick up another career path.
So either you combine careers or you live them one after the other. I think if it works for you and one career does not affect the other, why wouldn't you try.
Best of luck.