Depends if you really need a 2nd 1D Mark IIn. If you're constantly lens swapping, then it might be a good idea. If you really must have a backup camera, just in case your main one goes down with some fault, then again, it might be a good idea. Since you're not using the 1D and 30D, what are the reasons why? Ergonomics? Performance? Weight? Quality? A bit of all of the above? Will a 2nd 1D mark IIn fix this urge? True, if you do buy it, it'll hold good resale value should you decide that you don't need a 2nd body sometime in the future.
Think carefully, maybe there is a lens you might want instead 
If you're shooting football, then maybe a used 300mm f2.8 IS with 2x TC will be nice (for a very similar price to the 1D mark IIn setup).
Dave
