The message
from Ian Stirling contains these words:

Doesn't everyone use two spanners? Perhaps that's where all the people
who've been having compression joint fittings are going wrong, they're
just using one spanner!

Two spanners helps - but you've still got an unbalanced torque left.

Mitigated with offset spanners. Anyway, it was the self limiting by the
user of the applied torque to do up the fitting I was wondering about.
With only one spanner even quite inept people will limit the tightness
because the other half of the fitting is starting to turn.

--
Skipweasel
Pay no attention to that man behind the curtain.