I think Cliff and Mike are on to better fixes... First, keep the sensors clean. Every cook. A little grease on the sensor might cause it to run hot vs dry smoke residue cause it to be erratic.
Upgrade to a PID controller if you can and plan to keep the grill long enough to warrant it.
I don't expect the second barrel will help control temps enough to fix the issue if at all. It would slow down the fluctuation and it might(most likely) lower the over all temp.
It will use more fuel overall, may cause early burner failure. who knows for sure? but you gain cooking space.
And some of us do love modifying things.... So if you do it please post it! Think about having controllable vents between the two barrels....
This was just my humble opinion ...