If a baby received breastmilk from more than one women , would it promote increased immune system development ?

Unlikely: Donor breast milk is balanced nutrition and contains immune factors.However, these immune factors, iga, are transient and act to prevent germs that enter the digestive tract from attaching to babies cells to cause infection.The other immunoglobulins baby needs were passed during pregnancy. The donor milk just acts as a better substitute feed than formula.

Intriguing question: Theoretically the protective effects from breast milk may be additive since there will likely be more preformed antibodies in the breast milk when obtained from more than 1 woman. Another intriguing aspect is that this may also introduce more protective bacteria to the infant and steer his immune system away from allergy. I have not read any study on this subject however and thus it is a guess.