Might be misinterpreting you, but, to be clear, I don't see 'two spanners' and 'tab and slot' as two alternative ways of tightening a headset. They are complimentary.
Yea I understand that - I was just trying to figure out how important (if at all) the tabbed washer would be. I know the 2nd spanner will probably solve the problem - but I thought this tabbed washer could also be handy as a way of stopping the two nuts from moving as one unit (as you say).
That's how it loosens out on the road (I forgot to mention that) - as one unit. It's not the top loosening first, it's both together.
Anyway I will get a 2nd spanner and go from there. This retrobiking malarkey is turning out to be a voyage of discovery. Internal cam skewers, 2 spanner headset adjustment - the list goes on.
Thanks for the advice as always.