I own a pair of the Blackburn Neuro 6.0, each with the second-bike unit. All four bikes are covered that way. I did it this way because I like the higher-end units. These have cadence, HRM and altimeter. The second-bike kit is $45 compared to $225 for the whole thing. If I used cheaper units, I'd just go with multiples.

Anyway, other than getting confused sometimes because one bike has different sized tires than the other, it works fine. It figures out which bike it's on automagically, and I have both units synced to the same heart strap.

EDIT: Oh, and digital wireless is the way to go. No interference.