Bonding the bars to an internal sleeve, you'd have to run calc's on the stress levels, then find an adhesive with the necessary strength and elasticity, and further you would have to assure a constant adhesive thickness, so that the stress levels would stay in check within the elastic range of the metal.
Here again, it's the torsional loading that might really require a very special adhesive in this case, to prevent a typical progressive failure in the bond that that then moves axially along the entire length of the bonded overlap region.
So an over-generous length of overlap is no guarantee of reliability if the bond failure can progress axially due to torsional flexing of the sleeve and handlebar tubing.

And any time that you're using a high-performing adhesive in a highly-stressed application, just as with welding, the process really needs to be developed through the testing phase to assure reliability.