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Dried yeast (well, all yeast really) loses it's viability over time, but it's not like it falls off a cliff and dies. It's like an exponential tail off, so in reality it's just under pitching.Yeah probably should have kept it but needed two anyway. Do you always do a starter, I guess that would help to get the ball rolling with out of date yeast.
If you don't have time to make Starter, then you can always aerate the wort (if that's not your normal procedure) as that'll give the yeast a leg up. You can give it a further leg up (I think, but this is where my microbiology knowledge runs out) by giving it another aeration after 6, 12 or 24 hours.
The idea behind this is that you've underpitched (due to "out of date" yeast losing a lot of it's cell count). It'll start multiplying in the growth phase, but additional oxygen then enables the yeast to generate more sterols which means it can multiply more times to get you back up to a healthy amount of yeast for the rest of fermentation. That's what I understand anyway