Great idea, but it doesn’t seem so currently. I’m running dual sims as of today and it seems the DND setting is same as usual, just a “whole phone” feature rather than per-line.
Dual SIM is still a very ‘young’ feature for Apple. I think we’re going to see a lot of good stuff in the next few months as 12.x is updated (and hopefully not have to wait for iOS 13 for improvements).