Even if dmg split percentages are not equal to armors split percentages (but should be equal than sword slice a robe) - both armors will be shredded almost at the same time. Its not interesting. Not tactical. Its the same to have just 1 armor type.
I don't see how, because the typical enemy does not have 500/500 Physical and Magical armor, but more like 350 P / 650 M.
There certainly are some enemies which do have the same amount of P/M armor, but generally, they are different most of the time.
Right now, if I have a party of 2 Magic-damage dealers and 2 Physical-damage dealers, and I attack an enemy with 350 P / 650 M armor, the Physical damage dealers will break through the physical armor first, and the Magic-damage dealers aren't going to contribute to killing or disabling the enemy until they chip off a lot more armor.
Even only dealing 25% of their damage to physical, which then breaks through to vitality after physical armor gets broken, that would be an improvement to at least feel more effective.
We already have virtual durability - that numbers of armors on every item, and we could brake it in battles dealing dmg to gray and blue bars.
I think I still don't understand. Is what you are talking about different than the current system of breaking blue and grey bars?