This may not be what you’re looking for, but if you select your bottom voice (the red notes in your picture), then change them into triplets, both voices will be correct.
Thanks, Stephen, yes, it helps, though there is one more step to flip the tuplet number.
The problem is: the notes are moved to the other voice, but the triplets have to be duplicated. After a bit of experimenting, I found the following procedure, that doesn’t require repairing damage from lost triplets afterwards:
But TBH, I also think Dorico could handle this a bit more intuitively. The tuplet notes are not really quavers, i.e. eighth notes. They should be regarded als ‘twelfths’, and retain their value when copied or moved around.
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Completely agree! This has caused me much misery when I’ve copied a passage and didn’t notice until later that I didn’t copy a tuplet correctly. Changing this behavior is probably the top item on my wish list.
Yes, Pjotr, 
with a shortcut for Paste Into Special Voice this could be quite quick solution for now.