Hello,

the cable you describe converts only from V-Gate to S-Trigger, but not
the other way around.

A S-Trigger to Gate converter looks a little different. As example check

http://www.xmission.com/~dingebre/Trigger_Converter_4x12_1.1.pdf

The circuit in the most left row is the Gate->S-Trigger as you know it
from the Doepfer page.
The circuit in the second row is S-Trigger->Gate

As you can see, you need supply voltage for this. So the best advice is
to build this circuit either as additional Output into the MS-20 or as
additional into the DE.

Florian