In a back mount double tank setup with a manifold, the SPG usually is on the left-hand post, I believe. And you breathe the tanks down simultaneously. Should the right-hand valve fail, you close that valve and the manifold and you can breathe the left-hand tank down while you end the dive and ascend. You can keep monitoring that tank pressure, since the SPG is on that first stage.
But what if the left-hand tank, first stage or valve fails? You close the manifold and the failed left-hand side and breathe from the right-hand tank while ascending. So you cannot monitor your pressure and have to rely on the last known pressure and an immediate end of the dive. Is that it? No redundancy on the SPG then? Or am I missing something?