The phrase you might want to use is diametrically opposed, which literally means at opposite ends of a diameter, which is a straight line. Unfortunately (?), it also has an idiomatic meaning, but as I don’t think your holes dislike each other.
Alternatively, "every" is ok if it follows your existing convention. In theory, you could have more than two holes spaced every 180 deg, they’d just be co-located, so only two holes need drilled. I’m curious as to why you wouldn’t just draw the holes with a dotted straight line (or marked/labelled angles) between them. Two points can only define a straight line on a flat surface.