You have the 3.1 upgrade? That was supposed to mitigate the heat creep that was common with the Dual Extruder V3. It would appear that LulzBot’s 3.1 fix isn’t good enough for you.
On the V3, the shared heatsink had, IMO, poorly engineered cooling flow. The solution that worked for me is https://www.thingiverse.com/thing:3213878. I was able to get the stock fan to attach with some careful rerouting of the existing wiring. There are also higher CFM 40mm fans that can be used if necessary.
Other things to try are lowering the nozzle temperature, use an external fan to blow on the tool head (PLA loves cooling so this shouldn’t affect your prints). You may have to reduce the speed as well as the temperature (giving the filament just a little more time in the hot zone).