You make it sound like having a multi-GPU setup's performance limited to whichever vendor's card is slowest for a given scene, featureset limited to the intersection of card features both could support reliably, on drivers never meant to work in concert with their competitors, with a PC installed with two proprietary driver packages with separate update methods and schedules, drivers that frequently have problems with conflicts with older versions of themselves, with different .NET or other environmental requirements, on hardware that was not designed/developed/tested in the presence of a competitor that frequently has problems with differing hardware from the same vendor, with low-level differences in execution behavior, little history of cooperative implementations, multiple render paths in an engine that were never designed/coded/tested to run simultaneously, in a platform that has at best problematically supported switching between one GPU or the other in mobile, between vendors who have every interest and an ongoing history of getting in each other's way could lead to undesirable outcomes.
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