split secondary beams

Inputting a simple figure of six beamed sextuplet sixteenths. I want to split the secondary beam into 3 + 3. In Engrave mode I went to the properties panel, clicked on the split secondary beam, and all the secondary beams were broken, i.e. producing a monstrous looking 1+1+1+1+1+1 secondary beam. This was on the “auto” setting, but clicking through the other available options made no difference.

And suppose I want to split the secondary beam as 2+2+2? Or even 4+2?

It should be as simple as selecting the fourth sixteenth note in your sextuplet, right-clicking and choosing Beaming > Split Secondary Beam, as shown here: