roof: putting on a 2nd layer of plywood -- good? bad??

[applesusan]

In the process of tracking down/repairing some leaks in a relatively newly shingled roof (5yrs), we discovered that several spots on this part of the roof were quite soft. We thought it might be rot and began ripping out shingles/felt. Only a little bit of the wood has been exposed so far but that isn't rotten. It is just that where two pieces of plywood come together and aren't near a rafter, the wood bends ... a lot. One guy suggested we could lay down a second layer of (thinner) plywood on top of the first in a different pattern to give the roof strength instead of ripping out and replacing all the old plywood (assumng none is rotten). Does any one know if this is a good idea, a bad idea, or....
thanks

[stoneyboy]

applesusan,
If you can get to underneath of the existing ply you could add noggin strips to support the joins.
If not adding an extra layer of ply is a good idea but will entail a complete roof strip.
end