User blog comment:B214/Volume 22 spoilers/@comment-213.120.234.106-20160720102919/@comment-24.113.252.213-20160720203008

It could be that Issei gave his first to Rias, then Akeno. And the reason Akeno had a child first may be that Rias (being a Pure-Blooded Devil) had trouble having a child, and Akeno (being a former Human Hybrid) had a child more easily.

Devils have lower birth rates than Humans. So it makes sense that Akeno would have a child first if they did it with Issei around the same time (no matter which order).