It depends on the arrangement, your question is not clear.
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If Amy offers Bob a seller-financed mortgage, and Bob pays Amy principal and interest, Bob can deduct the interest. It doesn't matter for Bob's taxes where Amy got the money (another loan, or something else).
However, for Bob to deduct the interest, the loan must be a real mortgage -- that means it must be secured by the property so the holder of the mortgage (Amy) could foreclose and take the house if Bob stops paying. Securing the loan usually requires that Bob signs a lien which is filed against the property at the county clerk's office (this is sometimes called "perfecting" the mortgage). If the loan between Amy and Bob is not perfected (not secured by the property) then Bob can't deduct the interest because it isn't a real mortgage in the eyes of the IRS.
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On the other hand, if the bank has a mortgage to Amy, and Amy sells the house to Bob, and Bob pays Amy and Amy pays the bank, then Bob probably can't deduct the interest. However, this is complicated. The problem is that even though the loan is secured by the property, Bob's interest is not secured, it is Amy's interest that is secured. And in fact, if Amy sold the house without transferring the loan, the bank will probably foreclose if they find out, and Bob would be unable to get clear title in the first place.
