How can I preserve refrigerated cashew / almond milk by at least 2 months?
Simple: you can freeze it. Nut milks, especially homemade versions, can be frozen successfully. It doesn't seem to be a recommended practice according to commercial producers, but I would guess that's more to do with the emulsifiers and thickeners (typically lecithin or xanthan gum) used to improve the texture of commercially produced nut milks.

Assuming your freezer is properly set, this will remain safe for as long as it's kept frozen, although you might notice a decline in quality after extended storage.

The freezing and thawing can cause the nut milk to separate unevenly (making for a less pleasant texture) but this site suggests a quick run through the blender to help re-emulsify the milk after thawing.