This is a good question.
When I am technical diving I use two computers, they are essentially identical, and they are both wrist mounted. If I am on a rebreather, one of them is integrated to monitor my PO2 and the other is independent so it will calculate my decompression obligation based on a pre-set PO2. Normally they are within seconds of each other in their calculations.
Of course, that is probably not type of diving you were referring to in your original post.
I don't particularly like console mounted guages, so this isn't much of an issue with me. I wrist mount everything I can. But aside from the personal preference aspect, I also think wrist mount makes sense for redundant computers for this reason. You should check your redundant computer whenever you check your primary computer. If you have a secondary timer or computer clipped off on a D-ring or stuffed into a pocket, chances are you are not going to be looking at it. So what happens if the primary computer dies? If you have not looked at the back up, it might be dead too.
I realize that the chances of both computers crapping out on the same dive are remote. But I once had all three of my lights in a cave fail on the same dive, and they all failed for different reasons. What are the chances of that? But here's the rub, earlier that day, one of my dive buddies had all three of her lights fail during a previous dive.
My point is, perhaps it is better to consider your computers as two redundant computers rather than a primary and a backup. And if that is the case, then they should perhaps both be mounted in a similar fashion.
Jeff
Heyllo! 
