For me, the "limit" depends on arrow flight. If they both fly well then use both.
I mix it up on purpose for the following reason:
If you pick up a stone you've never held and chuck it, without needing to practice you will hit what you're throwing at given it's within range of your ability to accurately throw that stone. Pick up another one, same thing. Somehow our brains compensate for the weight and shape of the stone, and we throw it without thinking. For arrows, I've found that if I don't over-think the shot and I trust that the same part of my brain that knows how to throw a particular stone will also know how high to hold for the shot given an unknown arrow weight, I hit what I'm looking at.
Just a theory, but I think exercising that trust and instinct is what makes for a good traditional archer and is the reason I love this sport so much - it's not about the gadgets but more about the feeling. Go with your gut.