For now, I've figured out something that works for me. Basically, I keep a dedicated time carved out on my calendar -- my hour of lunch during the day job is spent editing on Uncertain Heirs, and my 8:30PM-10:30PM time block is also spent editing on Uncertain Heirs. Those are my absolutely writing times, dedicated to nothing but writing. Of course, given a good day, and no where else to be, I may work from 5:30PM on, so I'm free to write on Uncertain Heirs whenever I want.
The thing is now, with this other story vying for my attention, I've had to figure out ways to appease it while still making my (self-imposed) required writing goals. So, I've come up with a way that makes sense. This once-abandoned story is a side project, a pet project, if you will, and I have to give it attention like any pet. When it howls and insists on playing ball, I turn away from Uncertain Heirs and write on whatever scene had come to the forefront of my thoughts. I write, and write, and write until the pet's worn out, until it doesn't want to play anymore for the night. Then I put it to bed at 8:30PM, and get back to my block of time dedicated to finishing Uncertain Heirs.
If I'm out and about even, and I'm outside my lunch block or out to dinner with friends, I scribble what notes I can on the pet, so's I can save them for another time. Writing it (despite being out of sequence) when I can makes it so much better for me when I can lay it to rest for the night and focus solely on Uncertain Heirs.
So far, this is the best method I've discovered, but I realize it only works because it's only two competing projects. A few months ago, I wanted to work on everything at once (short stories included), and I ended up freezing, unable to work on anything. It was only after I tried focusing only on Uncertain Heirs that everything else, save the once-abandoned story, calmed down.
I can handle two, just not twenty.