Here are some answers I've confirmed with Chris:
Are you saying that everything the first two tracers trace in gets added to the cube?
As in player A traces their trace, and it goes in (kinda how it is now tbh, just nothing spawns from it) Then player B traces a completely different trace, do both traces go in?
Do they both spawn stuff?
As long as Relics require only weight 2 to spawn, then yes. With Artifacts, no, because they still need weight 3 to spawn. Like annkri observed, Scythes may need to be more vigilant on Level 2, or we'll have to consider changing the spawn threshold; though like Nseraf observed, "last Scythe wins" makes this much less dangerous to experiment with than it would have been a year ago.
then if they both spawn stuff, when player C puts their tiebreaker trace in, does stuff automatically just disappear?
Yes, anything spawned from a segment that was only added by Player A or Player B would despawn if Player C didn't agree.
And then how does this affect player 3? if they both over and undercolor if a and b have put in drastically different traces?
Player C, acting as a tiebreaker, will get points for agreeing with either Player A or Player B. If Player C's trace is itself wildly different from both of the others (either because none of the three traces agree or because A and B agree but C does not), then they will be considered wrong, but that's not much different from what happens to Player C now.
I suppose you could argue that changing things this way treats TBs as less valuable, increasing confidence in the next two players, but I'm not sure that's bad. It may just be that we've been wrong to imagine that the first trace from an enfranchised player is sufficiently likely to be truthful such that it should be presumed truthful; one way to guarantee a more sufficient likelihood would be to raise the TB/enfranchisement threshold to something excruciatingly high, but even super talented players can experience a 10-15% accuracy drop sometimes, and we can't inhibit cell growth too much. So as we tweak our system, we may have finally discovered that although the ability to TB should still be special, practical limits on its specialness also create a situation where we need to trust the next two players a little bit more than we have been.