How would this reduce taxes? If I normally charge 20k for widget Z but only invoice company A 10k because they will see me widget B for 10k and we trade widgets, there is no taxable event. If company A was willing to pay the 20k instead obviously I would rather have that even if it creates 10k taxable income because profit.

Investor fraud is much more likely if neither company actually needs each other's widgets and it's just to pump revenue.