We present a study on the quantum transport properties of chemically functionalized metallic double-walled carbon nanotubes (DWNTs) with lengths reaching the micrometer scale. First-principles calculations evidence that, for coaxial tubes separated by the typical graphitic van der Waals-bond distance, the chemical modification of the outer wall with sp$^3$-type defects affects the electronic structure of both the outer and the inner tube, which reduces significantly the charge transport capability of the DWNT. For larger spacing between sidewalls, the conductivity of the outer wall decreases with increasing functional group coverage density while charge transport in the inner tube is equivalent to that of a pristine nanotube. Additionally, chemical attachment of CCl$_2$ onto the outer DWNT sidewall barely affect the conjugated $pi$-network of the double-wall and charge transport remains in the quasi-ballistic regime. These results indicate an efficient route for tailoring electronic transport in DWNTs provided inner shell geometry and functional groups are properly chosen.
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