Now this kept me very curious, wanted to test my theory by simulation and found it a great opportunity for learning Excel. I created a simulation that would show how much damage 32 shots would do over time when chaining critical Overkill shots. For the gun I thought of a Jakobs sniper with 3000 normal damage and about 11K crit damage. Which shots are TF, and what health the enemies have is randomised on each run.
So when we give the enemies 20,000 health (± 20%), we’ll quickly get to the point where we always hit the cap, which is normally x5, but x3 on Two F4ng shots that kill the enemy with only one projectile, which always happens at this low health. It doesn’t really matter though because every hit is a kill either way.

Raising health to 30,000 ± 20% causes problems for Two F4ng. Without the skill, once the overkill cap is reached it firmly stays there. TF shots however mess with this and cause drops out of the chain, from where it has to recover to reach the cap again. Sometimes this causes it to not even kill an enemy on hit, resetting excess damage to 0:

This fluctuates a lot on every run, some are luckier, some less – but about 80% of the time, TF caused a decrease in average damage.
However if we raise health further to 40,000 ± 20%, we’re reaching a point where we can’t really chain Overkill anymore, needing 2 hits for most enemies. Now TF becomes good because it’s mostly a DPS increase. Thanks to enemies with more health, you’ll much more often see TF doing full overkill damage with both bullets now, which raises the damage cap of both bullets combined to 6x regular damage (TF capping at 6x damage four times here, normal capping at 5x two times):

This also fluctuates a lot, but on average it’s clearly Two F4ng that’s doing better now.
So if Two F4ng is better or worse depends on a lot of things, among them a lot of chance. However at the sweet spot where you can just consistently chain Overkill shots, it seems to be rather unhelpful.
So should you take this skill? I think yes, because most of the time it’s just a great DPS increase. It’s only a certain scenario where it doesn’t synergise well with Overkill.