FeynAmpDenominatorCombine[expr] expands expr with respect to FeynAmpDenominator and combines products of FeynAmpDenominator in expr into one FeynAmpDenominator.
Overview, FeynAmpDenominatorSplit.
FAD[q] FAD[q - p]
ex = FeynAmpDenominatorCombine[%]\frac{1}{q^2 (q-p)^2}
\frac{1}{q^2.(q-p)^2}
ex // FCE // StandardForm
(*FAD[q, -p + q]*)ex2 = FeynAmpDenominatorSplit[ex]\frac{1}{q^2 (q-p)^2}
ex2 // FCE // StandardForm
(*FAD[q] FAD[-p + q]*)