Ok for context this was a while ago; I wanted Dog princess in my Party and I was already on Wyvern Princess' Route, the Wiki said i can control DP's Violence with the beast choker and that's what I did, so DP joined my Party and I was able to finish to finish WP's route. I am now trying the same thing with Dragon Princess I, but DP's route hijacked the Dragon Route; my questions are: can I have Dog Princess in my party and finish the Dragon or does Dogs route take priority? And also, can I have Dog princess in my party and do Any other princess route or was the first thing I did some bug or glitch?
Yeah as far as I know, you can only do one route per princess (excluding reignless princesses like mimic, dog, moss etc.)
Since you did Wyvern Princess's route, you cannot do any other route until you do new game + or just switching over to a different save. I presume you saw the warning when you went back to wyvern princess after fighting her right? Also side note, if dog princess isn't allowing anyone to join your party just drop her off at the tavern, pick whoever you wanna pick up and then bring back dog princess. So if you wanna do DP's route I believe you can't do WP's route, vice versa. If you wanna know how to do NG+ and keep all your scenes with bab, you can go to the kingdom below, go to the ruins area with your gas mask & go to the turtle shell with the lights on and enter that. I recommend getting mimic princess before you do that though. To sum it all up: You can only do 1 route, unless you're doing a princess route that's reignless.