Can I blend high-protein (baking) flour and 00 flour to approximate plain (AP) flour?
First, in both the US and Australia, 00 flour has about the same protein content as AP flour, so you can use it instead, keeping in mind that it's more finely milled. To answer your broader question, yes, you can definitely combine flours together to approximate AP flour (keeping in mind that protein content in AP flour will range from brand to brand). In the US:


cake flour has low protein content (7-9%)
AP flour has medium protein content (10-12%)
bread flour has high protein content (13-16%)


In Australia and other countries the flour percentages may differ from my chart above. I believe Australia has lower protein content in the US in general for AP flour, and 00 flour is also lower, but within the same range as AP flour there.