Have you tried the beam adjustment tool?

… The Stem Length tool in Advanced Tools …

This is a misunderstanding.

For adjusting single (unbeamed) notes you can use the Stem Length Tool.

For beamed notes, use the Beam Angle Tool.

By The Way:

I do not know of a (sub-)tool called Beam Adjustment Tool.

Thanks for your replies Mike and Peter. Sorry, I was inventing my own names for the tools. The problem is that neither the "Stem Length" tool nor the "Beam Angle" tool have any effect on the second (downward-pointing) stem. I've even tried changing every different stem length setting in the "Document" menu, but all to no avail ....... so I'm still stuck !

I suppose that you created the extra stems with the Double/Split Stem Tool - right?

As Far As I Know, the length of such extra stems can not be individually adjusted.

You can, in Document Options - Stems, set the general value for Normal Stem Length.

That value will also affect the extra stems.

But you can not adjust the extra stems individually.

However, you have full control if you instead add the extra stems in Layer 2:

Great. Then it is the new layer that's the solution then!  A little more laborious than a simple Beam Angle adjustment, but it will definitely do the trick. Many thanks again, Peter