Thanks for 'confirming' my nightmare MT!
Your suggestion of a separate fuel gauge fits if just one gauge were affected, but despite my best attempts at wishful thinking I don't believe that the cause being sender units in both tanks is likely, particularly as there is a relationship between the misbehavior of both.
Can anyone tell me how many arms & legs (approx $ value would be good) I'd need to sell in a worst case scenario where the instrument cluster needed replacing. Would going to a Nissan dealer or an auto sparky be the best course? I'm guessing Nissan would just want to replace, but how likely is it that an auto sparky would be prepared/able to fault find/repair on the back of the dash cluster.
Living with the issue locally isn't hard, but I must admit I want to be able to trust the instruments for touring & especially any long desert trips.

Sorry if this is a hijack Rafa, but I'm assuming it may also be helpful to you.