Lets say you are working on one novel / short story. As you work you suddenly get an inspiration for another piece of fiction. You feel it's a brilliant idea and you should start working on it right away. Your brain tells you to focus and finish your first story, but your heart goads you to go ahead and develop the idea while it is still fresh. How practical and feasible is it? Are you doing justice to your first story? Has anybody been through this? What should I do in such a case?
Block and outline it in a writer's diary or even in a new word-processor document, so you have the idea to come back to once you've reached a good stopping point with your existing novel.
It's not a bad thing to be inspired to do something new. The danger, in writing at least, is that if you follow your inspirations you could end up never finishing anything, being forever pulled off-task by yet another new idea and eventually losing track of or motivation to finish your original work. This is the reasoning behind the creative diary; anything you're not actively working on or that doesn't fit in your current story can be set down in a scratch-pad format to come back to when you need new ideas.
Another point; your publisher, if/when you get that far, is unlikely to want to initiate the editing/publishing/distribution process on more than one book at a time, and you're not going to want to try to promote two books at once unless there's some sort of overarching glue like a story arch or a storytelling series (and even then you'll want to get one title out there being read before you follow up so you establish a progression for reading them).
To further what KeithS said, I agree. I would continue your first work, but write the idea down somewhere so you have it for later. I keep a notebook dedicated specifically to book ideas, so when I come up with one, I write everything down about that idea in the notebook, then get back to my current WIP knowing that I can come back to the other one. I would be sure to write everything about that idea down - every little speck of an idea - because that will help you get back into the excitement later.
That said, I think it also depends on what stage you're in with your current WIP. I've been told by several authors that they keep three works going at all times. One in the plotting stage, one in the writing, and one in the editing. I haven't yet been able to master three, but I do have two going pretty much all the time. It's easier to shift focus to another project when you're not having to put the same efforts into it as the other one (i.e. if you're stuck on writing, you can always switch to editing to clear your head).
I think user KeithS has a great point about finishing what you start. That being said, Ray Bradbury used to have 19 (I believe) typewriters, all with different stories going on each one. He'd move from one to another and write whatever story motivated him at the time.
On a side note, I'd LOVE to be able to do this. So, is it practical and feasible? Sure is. If Bradbury could do it with typewriters, how much easier is it with software. If you're the right kind of person to do it, you should do so.
Could be a lot of fun. :)
On my computer I have a folder called "books". In that folder are about 30 other folders, each one containing the draft of an unfinished book. In addition, in the main "books" directory, there are about 50 text documents each of which is a treatment or other material which might serve to get a book started. My plan is to direct the executor of my will to print all that crap out and bury it with me when I die. Does that answer your question?
Piers Anthony described that in "authors notes", and how his style changed as technology improved.
To make a long story short, when an idea intrudes log it in another file for that purpose and then return to the current job.
By "log" I mean append a note to a log file, as opposed to editing the new information into the body of notes. A different task will be to go over new logged notes and work them into the organized structure, musing and giving the whole attention to this story.
An intrusive idea is like a sneeze. Get it out, wipe up, and don't get your current work pushed out of your mind.