Hi. Peter;
I am not talking about two subfloors, I am saying that an adequate underlayment could be installed over a CDX plywood.
In today’s World of flooring Manufacturers Specifications for installations, I would reccommend adhering to their requirements, because they pretty well dictate what they want for underlayments.
As you might already know, OSB is not approved for an underlayment for VCT and Linoleum products, and have to install another underlayment. Now, as you can see, you have a sub-floor of 3/4" Advantec Sturdie floor and have to install a different underlayment for the floor finish selected.
Years ago, it was customary to have to layers on your floor framing system. 3/4" t&g boards at a diagonal and plywood underlayment of various thicknesses.
Check with Floor Finish Manufacturers, the one layer sub-floor/underlayment that was installed might not be adequate for floor finishes.
Marcel 
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