That's an interesting idea.
If King Arthur provided more information on its flours, we could almost predict what would happen by combining the KA bread flour and the KASL in the ratio you suggested. KA provides a detailed spec sheet for the KASL but I haven't been able to find one for the KA bread flour. But I believe both are made from hard spring wheat. The KASL has a protein content of 14.2%, which is higher than almost all high-gluten flours, and the KA bread flour has a protein content of 12.7%. I would bet that the differences between the two flours beyond protein content are not all that great. If that is true, then a 50/50 mix of the two flours would have a protein content of 13.45%. That's about what several high-gluten flours have as protein content. Tom Lehmann recommends 13.5-14% or more for his NY style dough recipe. So, the mix you suggest would work.
Moreover, I have made the Lehmann recipe with KA bread flour and it makes a good pizza. Using the two flours should be even better and not far from using only the KASL. Maybe someone can give your suggestion a try.