How to write a thank you note to two separate people

I recently received a gift from my fiance's grandma and great aunt. They pooled their money to buy us a nice gift and I want to know if I should send them two separate thank you notes? Or is it appropriate to send them one thank you note, since they are close and talk often? If I send them two separate notes, should I acknowledge that they pooled their money? If so, how would I do that?