If you mean how to break the beam like that, I got as far as this, which is accomplished by splitting the beam on both notes. Edit>Notations>Beaming>Split Beam

But then I find no way to control the length of the beams. Does Dorico have something that corresponds to FInale’s beam extension tool?

It looks the same for me as in the attached picture
@soundware If that looks the same to you as in the example you posted, then I guess your problem was how to put the lower notes of the octaves on the lower staff and you have solved that issue. Excellent.
However, for me the problem is the broken beaming and I wonder if anyone has a solution.
Here is my attempt to create the broken beam:

In the second beat of the lower voice is an eighth note triplet containing a dotted eighth B♮, an eighth B♮, and a sixteenth C. After selecting the first note in the triplet, Edit > Notations > Beaming > Beam Together is executed. The second note in the triplet has its stem, notehead and ledger line hidden in engrave mode. Then the spacing of the notes and the angles of the beams are adjusted.
This is phenomenal @johnkprice !!!
Truly ingenious, @johnkprice What a great lesson in this special use of the beam together command that I know I will make use of on occasion. Thanks so much>
However, I request the ability to adjust the length of all beams. Handles appear when a beam is selected in Engrave mode, but these only allow vertical adjustment.

