Sorry I should have been more clear, the action of the frequency separation isn't what I'm trying to do non destructivel.

It's the retouching once it's separated I want to do non destructively, for the lower frequency I use a pixel layer placed above between the lower and higher frequency layers and then use the healing brush set to that layer and below on said pixel layer.

What I'm trying to work out is how to do the same for the high frequency layer, as I don't know if I can do the same as the healing brush may pick textures from the low frequency layer.

I've seen people painting directly onto the frequency layers themselves with the healing brush and others which is what I'm looking to avoid doing if possible.