I just cast some 690 RB to load in magtech brass and was thinking about trying some double ball loads out, I was thinking about trying 60-70gn, a fiber wad, RB, cushion wad and another RB with an overshot card.

Does that sound safe, I've seen fellas on YouTube load two RB over 90gn in 20ga muzzle loaders, but I don't know for sure what difference the brass case might make if any.