Re: New sheathing on top of siding?
You should be able to shingle right over the T1-11 siding, obviously adding the necessary roofing felt, etc. The only thing you will need to do is replace any damaged materials beforehand. If there is bad siding, replace it, if there is bad framing, replace/repair it, as goes the sub-sheathing if necessary and filling in areas where no siding exists. You need to have a sound shell with a single plane surface, then you can install your siding shakes over the top with no further sheathing necessary.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!