So this thread is just a log of progress for this upgrade.
Looks like initially the table for wiring harnesses ( https://www.lulzbot.com/tool-head-adapters ) is incorrect, you additionally need EL-HR0064, in addition to EL-HR0061 and EL-HR0065.
65 is the internal add on, which connects the board to the pinsets, for the additional blower fan, presumably the second toolhead/extruder/internal connection with the TAZ 4 came with that secondary internal connection. I haven’t opened it up yet, but I feel this is probably not true either.
61 is the adapter at the carriage, for the existing taz4 wiring harness to connect to the new toolheads all-in-one connector (v2c).
EL-HR0061:
64 is the part that is not on the harness guide for TAZ4 and is necessary to connect the pinout from the board box through the molex arcade plug, through the existing wiring harness (missing these two wires for the fan) to the carriage end of the wiring harness. Luckily we had this leftover from a different install. IDK if the EL-HR0064 comes with pins to put into the molex plug, or a fan attachment connector, either way the harness guide is incorrect as this set of wires is missing on it)
I’m not sure which “version” of the taz4 I’m working on, but I have worked on several, and none of them have this included as stock (EL-HR0064 or EL-HR0065).
Old and New wiring harnesses (old is the one missing 3 pins and is from a taz 4, the new is supplied with the dual toolhead)
Regardless of this, the HEX all metal toolheads print great once you get them dialed in (pretty simple) so this dual hex should be amazing! We built a hex and bought an assembled hex, and the built hex head is performing the same as the stock-assembled all metal hex toolhead.
Will update as I finish each step so you have a user POV of the install process. I didn’t see any existing threads like this. Ask any questions you’d like, I probably can’t answer them but someone can! 
