How many long stories(you know, novellas and novels), have you written before now? That's a factor I think.
I'm new to writing anything longer than a short story, or more tidy than a blog(very relaxed in my blogging). I wouldn't begin to start on any of my other ideas. It's because I want to finish this story. I know, sure as shootin', I'd get sucked in with, in my case, both stories, and more than likely, I would end up with two, mediocre at best, unfinished stories. Best case scenario, one, finished, good story, and one, crummy, unfinished, story.
That's just me though, I've done it with painting, stands to reason I would do it with writing. My best painting, the one that gets the most positive responses, and sparked the most debate, I painted just that one painting, and didn't have all the burners on at once.
You can always round-robin the projects. Finish the first draft of each idea seperately(take notes all the time, sure, heck sketch if that helps, or use the voice record feature on your cell phone), just so you don't feel like you're stagnating. Also, I'm thinking, your first draft will be easier to edit with fresh eyes, and working on a different idea will give them a break.
Just remember, I'm not telling you what to do. I'm probably even more lost than you are. Just tossing out ideas, which may be helpful.
