If your subwoofer cable is (well) shielded, wouldn't it be a good idea to make sure that the splitter is shielded (well) too?
One that I found that seemed to be a good balance between price and confidence in the quality was an Acoustic Research Pro Series II PR142 (or PR142N). This is the 1F to 2M version, I think PR141 is 1M to 2F if you need that by chance.
I didn't purchase any yet because I am waiting to figure out what other cables and wires I need to save on shipping, but the best prices I found on this Y-cable was $12.70 at electotronics.com (but shipping is high for small orders) or $14.32 at homedecorworld.com (with lower shipping charges). The low price, high shipping place is probably only worth it if you order 3 or more cables (or other stuff). They are $30 at Partsexpress.com.
If anyone knows of quality shielded splitters for less, 1F to 3M splitters, or 1F to 4M splitters I'd love to know where to shop.