User blog comment:B214/Question and Answers 6/@comment-25072311-20190401190525/@comment-10330822-20190425045526

DxDlover. The DxD forms are essentially their "god forms", the "diabolos dragons". It's what happens when you mix Issei and Vali's power with the power of Ophis, the Dragon God.

It was never stated that there is a child limit, the fact of the matter is that Rias is a pureblood devil, so it is difficult for her to have children. Its for that reason the pureblood devil population is still in danger of dying out, and the Gremory family only has two heirs. The upside is that Rias and Issei are young, and they have hundreds of years to try.

Its not specified who is stronger, because both have crossed into Super Devil/God territory. Yes, they would need to fight to be certain, but things are looking pretty good for Issei being stronger.

Let's just say Issei finds away to use Ddraig besides just having him inside of the armor, which allows him to fight two people at the same time.