I've been so successful with using Exiftool that I'm thinking of some other possibilties. Your fault for giving us such a powerful utility. :-)

As Lightroom and Acdsee both save Raw settings to sidecar XMP files and it now seems withing the realms of possiblity to store both sets of data in the Raw file I'm reworking my batchfiles to copy required fields from these sidecar files to the raw files directly.

Originally your previous suggestion works well but leads to problems particularly when using Acdsee which doesn't retain any Lightroom fields.
Interestingly LR does retain Acdsee settings by copying -xmp:acdsee to it's database.

So if I were to use:

to copy the sidecar files I get into a problem if I do any keywording in Acdsee after storing LR Raw adjustments, simply because it's replacing ALL existing xmp data

So I came up with the following to isolate tags that I want to copy from any sidecar file:

Copyfrom.args currently has just a few test lines in it:

The problem I have is that I cannot get exiftools to copy the acdsee tag (line 3), I get an error
'no writeable tags found in dsc_2774.xmp'. I've tried several alternatives but with the same result.

This is confusing as the xmp-crs: field copies without problem and so do the other xmp-dc tags.

Acdsee's tag starts like this:

Do you have any idea what might be the problem?