The Rambam writes in the 10th perek of Hilchos Ishus (halacha 13-14) that if a man marries a woman, he is obligated to have sheva berachos for seven days. If one marries several women at once, he must have separate sheva berachos for each one for seven days – consecutively. The reason for this is because we cannot mix one simcha with another simcha.