The logic is:
* If one bike is not running right for unknown reasons then parts can be swapped from the running bike to find the problem.
* When your having trouble explaining to the mechanic what you want done on your broken bike/rack/etc. you can just point to the runner.
* It just simplifies things - arrived in town and need new tyres? Find them for one bike, you've found them for the other, too.
* Convenience of having a spare-spare-part on the other person's bike - if your spare is broken or is being used and breaks before replacements are available, then you can use the other person's spare.
* Only one set of manuals.
* Something goes wrong on one bike, you know about it for the other, and maybe how to fix it.
There's probably more reasons I forgot - it's a good idea
[This message has been edited by JamesCo (edited 08 September 2005).]