Merging Two Notes

[Will Oram]

OK, imagine this:

Between two polyphonic lines, there is a certain note that is shared 
between them, but of different lengths.
eg:
  _____
 |     |
*  *  *
|__|__|
   3

(Like my drawing? :)

lilypond is technically correct in spacing the last note of the two 
lines differently. For aesthetic reasons, though, the score I'm looking 
at combines them. I don't know what to call this phenomenon, so it's 
hard to look it up in the index/on the website.

Anyone know if this is possible?

Thanks,