Here you go!
Brahms Intermezzo - edited.dorico (469.4 KB)
It’s a bit of a fudge, but it it might just do the trick.
As shown in the second image, I added a note above each of the last two 16ths between the staves and then moved them down to become unisons. This had the effect of pushing one of the noteheads to the left of the stem. I then selected the noteheads on the righthand side of the stem and scaled them to 0% in Properties - this can be done in either Write or Engrave mode.
That was the easy part!
Once the slurs and diminuendo hairpin had been added, Dorico’s automatic collision avoidance kicked in and started moving things away from each other, as it has been designed to do.
To get the vertical spacing right, I had to just keep fiddling around with dragging the 16th beam up and down, re-positioning the slurs and hairpin, and using Staff Spacing in Engrave mode.
I eventually got there.
If you zoom in on the notes, you will see a slight protrusion of the stem, as indicated by the red arrow. This might (or might not) be unnoticeable when it is reduced to normal size.
One last thing - when I opened your file, Dorico informed me that it had been created in an earlier version of Dorico. The edited version I have put here was saved in the latest version (3.5.12). I don’t know whether your version will be able to open it.