It's been a while since I read Dokgo 2, but Dokgo 2 I think concludes around the same time Tong Vanity concludes.
Blood Rain 2 and Blood Rain 1 actually overlap for 22 chapters. Hence the reason In Bum and Shinsa talk about Kim Min Kyu in the prologue.
If you guys haven't already. Check out Tong Edge. I haven't read all of it yet, but it happens right after Tong Vanity. It's free on Toonmics.
IMO, Dokgo 2 isn't exactly a required read to understand what is going on in BR1 and 2. BR1, I feel lays out the story pretty well for someone new to pick up.