It is essential to time the insemination in conjunction with the time of ovulation for maximum success. The eggs remain alive for only 12 to 24 hours after release whereas the sperm remain alive for 4 to 5 days inside the women's body. Hence, the IUI must be carefully timed so that the sperms are available in the woman's body at the time when the egg is released (ovulation). Few studies suggest that two inseminations in successive days have higher chances of pregnancy than the single insemination whereas other studies show no such effects. The possible reason for the different findings is that if single inseminations are not accurately timed for ovulation, success rates would increase with a double insemination protocol. The bottom line is at least one of the two insemination needs to be correctly timed. There is no one size fits all, in case of fertility treatment so your doctor will design your IUI treatment after having your thorough examination.