FeynAmpDenominatorSplit[expr] splits all FeynAmpDenominator[a,b, ...] in expr into FeynAmpDenominator[a]*FeynAmpDenominator[b]*... . FeynAmpDenominatorSplit[expr, Momentum ->q1] splits all FeynAmpDenominator in expr into two products, one containing q1 and other momenta, the second being free of q1.
Overview, FeynAmpDenominatorCombine.
FAD[q1, q1 - p, q1 - q2, q2, q2 - p]
ex = FeynAmpDenominatorSplit[%]\frac{1}{\text{q1}^2.(\text{q1}-p)^2.(\text{q1}-\text{q2})^2.\text{q2}^2.(\text{q2}-p)^2}
\frac{1}{\text{q1}^2 \;\text{q2}^2 (\text{q1}-p)^2 (\text{q2}-p)^2 (\text{q1}-\text{q2})^2}
ex // FCE // StandardForm
(*FAD[q1] FAD[-p + q1] FAD[q1 - q2] FAD[q2] FAD[-p + q2]*)ex = FeynAmpDenominatorSplit[FAD[q1, q1 - p, q1 - q2, q2, q2 - p], Momentum -> {q1}]\frac{1}{\text{q2}^2.(\text{q2}-p)^2 \;\text{q1}^2.(\text{q1}-p)^2.(\text{q1}-\text{q2})^2}
ex // FCE // StandardForm
(*FAD[q2, -p + q2] FAD[q1, -p + q1, q1 - q2]*)FeynAmpDenominatorCombine[ex] // FCE // StandardForm
(*FAD[q1, q2, q1 - q2, -p + q1, -p + q2]*)