Let's not forget that the project was never initially intended to be a "duet" album. It was going to be a Fleetwood Mac album. They started calling it a duet album after it was clear $tevie was opting out. My point being, the material wasn't constructed with that specific model in mind.
I do agree, it would be nice to hear her more prominently on his songs. I suspect she might actually be in the mix on some tracks, but she's blended (too well) amongst the many layers.
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