Several "writing theory" books I've read, especially Writing to Sell by Scott Meredith, have made the point that (for many people, at least) one's subconscious often comes up with excuses---any reason that might do---not to work on whatever one is supposed to be working on, which for a writer is one's current WIP (as homework is for the student). (One writer, I think the writer of the introduction to Writing to Sell but I'm not sure, had "an imaginary monkey that cried piteously if not loved every hour on the hour".) And one common excuse is a new idea that looks (at the time) like a better idea. It may be, it may not be, but once you get as far into it a you are into your current WIP, Meredith said (though I paraphrase freely), your mind will produce yet another "better idea," so write that idea down (so you don't lose it) and keep working on your current story. And if you're really blocked, switch to something else that you're supposed to be doing (which, for the authors Meredith was writing to, three decades ago or so, was largely finding markets for your last story), so that your current story becomes the excuse your mind will give you for avoiding it.

I guess that if you haven't written anything else, instead of marketing, one can switch between two main stories.

Mmm ... I think that, if Scott Meredith is right, something like laundry, tidying one's desk, catching up on email, etc., would be better than a second story---on the theory that "inspiration" is a finite resource (though not, as other anecdotes from Writing to Sell make clear, so finite as writers tend to think), when you don't "feel like" writing, do something that you need to do sometime that you "feel like" doing even less .