User blog comment:Xhatefate/DxD Updates~/@comment-27083298-20160805125112/@comment-24.113.252.213-20160805180707

The two crimsons refer to Sirzechs (deep red) and Issei (true red).

Sirzechs has always considered Issei to be his little brother (in-law), and they team up to fight in this volume, so this title reflects that.