Sometimes...I would worry about frequencies cancellation or something of the sort, a simple summing circuit of two resistors, one in series for each channel and tied at the far end for the mono signal would be better. I think any value from 100 ohms to 1kohmns should be fine.
I have a Marchand electronic crossover kit and they recommend a value of at least 100ohms. I was using this for a single subwoofer by the way.
I remember reading an article at Rane some time ago entitled "Why not Y" or something to that effect, wherein they went into the reasons why using these 'Y' cables are not a good idea.
Just found the webpage:
http://www.rane.com/note109.html
