Can I mix all-purpose flour with high-gluten flour to make bread flour?
Yes, you can mix them at basically a 1:1 ratio to achieve a flour with roughly 12% protein content (mimicking bread flour).

Bread flour is milled from hard spring wheat, which has a higher protein content than the hard winter wheat used in all-purpose flour. Protein adds strength to dough and enables loaves of bread to rise high.

Mixing them together to achieve a roughly 12% protein content will ensure you don't end up with a bread that's way too dense/soft or rises far too much/little, basically.

There's no issues mixing flours, and in fact this can be a great way to create the mixture you need when it's sold out (such as during this pandemic).