SemiconEng, the term would be 'dualopoly'. Anti-trust law does not forbid dualopoly - or monopoly. (The best explanation of this can be found in the Appelate decision on Microsoft a few years ago, where Microsoft was found to be a monopoly and then they proceeded to distinguish between monopoly and illegal monopolist. The latter is a company which abuses its monopoly position by leveraging it to dominate other areas.)
Anti-trust law has been used, generally unsuccessfully, to attack virtual monopolies of more than one company. US law does not permit a collection of companies to act in concert to dominate an industry. The oil industry was accused of this in the 1970s, but it is very hard to distinguish between independent decisions reacting to a common market vs. collusion. Without a paper trail, it is near impossible.
Wink, wink; nod, nod.
In the case of Intel and AMD, no rational person would accuse them of collusion. The record shows just the opposite.
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