Using two bodies

How do people use a second identical body?

Do you use one exclusively, aiming to keep the other in 'new' condition, or do you use them about equally?

I can see pros and cons of each approach.

how about one set up for one type of shooting and another set up for a different type of shooting...
ie during a wedding I might have one set up for inside the church, and the other set up for outside....

Not quite what I meant - obviously for a wedding there's very good reason to use both cameras.

I was thinking more along the lines of a second one as a backup - but presumably not wanting to cart both of them around all the time.

ah..i get it.
I'd use the batteries from both, and alternate after each use, leave one camera in the bag put all the wear and tear on one, while preserving the back up.....a worn out camera is always a great reason to upgrade, to a newer better model, so I'd try to waer one out.

I have continued to tote around my istD as a back up, I have even gone out with the intention of shooting with a long lens on the K10D and a shorter one on the istD, problem is I love the K10D that much I cant resist swapping to the shorter lens anyway

As for back up bodies, I can count on one hand the number of times a camera has let me down over the last twenty five years but I always had two bodies with me anyway, albeit in the days of film they may have contained different stock.

As for a second identical body....are you that rich

In the days of film, a backup body could have different film stock in it, or maybe colour in one body and mono in another.

With digital, there is no longer any need for a second body as colour/mono/slow ISO/fast ISO is just a button away.

As regards reliability, in 35 years I have not had a failure in the field, so the chances must be very low. On holiday I would buy a K100D to cover if it ever arose. At a wedding, Sue and I both have a K10D so we would make do with one of them.

The final backups are out pocket cameras. Sue uses a S5i and I use an A20, not ideal, but serviceable in extremis.

I don't have an "identical body" but occasionally I have toted around the DS as a second body. One will have a long lens and the other a wide lens, long on the K10D because the SR and more rapid AF gives more benefit to the long lens (IMHO).