My house is a 70s style split level with soffit, ridge, and gable end venting. Right now there is only one layer of fiberglass batt insulation and I need to add a second layer (current r value is woefully low for my region). Due to some obstacles in the attic such as skylights, bathroom vents, and chimney, it is too difficult to climb behind to the rafters and add baffles in those areas so I've ruled out adding blown in. I've been told that rock wool would be a good way to go and would like to know how adding a second layer of rock wool would affect the fiberglass. The second layer of rock wool would criss cross the fiberglass and therefore rest on top of the trusses.

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Your plan sounds solid. Glass and rock are both quite inert and there is no reason to not mix them. Just make sure to not block airflow from soffits and keep the batts tight together.

Personally, I would use one or the other based exclusively on cost in your particular application.

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