FactoringDenominator is an option for Collect2. It is taken into account only when the option Numerator is set to True. If FactoringDenominator is set to any function f, this function will be applied to the denominator of the fraction. The default value is False, i.e. the denominator will be left unchanged.
ex = (x1 a^2 + y 1 a^2 + 2 a b + x2 b^2 + y2 b^2)/(a + b + c^2 +
2 c d + d^2)\frac{a^2 \;\text{x1}+a^2 y+2 a b+b^2 \;\text{x2}+b^2 \;\text{y2}}{a+b+c^2+2 c d+d^2}
Collect2[ex, a, b]\frac{a^2 (\text{x1}+y)}{a+b+c^2+2 c d+d^2}+\frac{b^2 (\text{x2}+\text{y2})}{a+b+c^2+2 c d+d^2}+\frac{2 a b}{a+b+c^2+2 c d+d^2}
Collect2[ex, a, b, Numerator -> True]\frac{a^2 (\text{x1}+y)+2 a b+b^2 (\text{x2}+\text{y2})}{a+b+c^2+2 c d+d^2}
Collect2[ex, a, b, Numerator -> True, FactoringDenominator -> Simplify]\frac{a^2 (\text{x1}+y)+2 a b+b^2 (\text{x2}+\text{y2})}{a+b+(c+d)^2}