only one machine is ever allowed to be plugged in at any one time, i.e. there will always be a machine safe from a power spike. Therefore data transfer between the two machines will have to done via a third storage device, which could just be a hard drive.
-nudone
Actually this is what I do with my laptop - I keep all important data synced with my desktop.
-Carol Haynes
Obviously, this depends on how you work and at what but don't forget to
keep the software itself as much in sync as possible — not just the data.
One continually develops habits and ways of working that improve productivity — a macro here, a little bit of code there, some software that's improved by a useful update, which in turn further changes work habits in small ways. Suddenly swapping to a machine that hasn't your own current conveniences and enhancements can add a layer of frustration that'll have you gnashing your teeth and ripping out hair. That's especially true in a small shop when attempting to service a deadline that's already been threatened by the principal machine going down. Just a few tiny examples: were your favourite file manger suddenly to revert to its out-of-the-box state, just how annoyed might you be? Step down from CS5 to CS4 in mid project — okay by you? Wrong graphics tablet driver?
For me, at home, this means a short session on each machine every couple of weeks or so.
Having just been through this, I realize that I should keep a log of the changes made to my main machine — I missed some. It all sounds very tedious but if one makes it a routine it's no worse than doing the dishes. When the crunch comes, you'll thank that previous you for spending an hour, then, to save you five minutes now.