You don't need to decant the first step if your pressed for time, but when you mix up the DME for your second step you should make the gravity a little higher to compensate for dilution from the liter of spent wort in the first step.
If you mix up another liter of wort at a gravity of about 1.065, it should dilute out to around 1.037. (assuming the first liter was mixed up at 1.037 and fermented out to 1.010)
As far as fermentation time goes, temperature plays a big role in speeding things up. If you can ferment your starter at around 90 degrees it will definitely be done inside of 24 hrs.
As for decanting, 8 hours might not be enough time in the fridge, depending on the floccability of the yeast. It usually takes at least 24 hrs to get the majority of the yeast to drop out of suspension, unless you are using something like WLP002, in which case it will drop out in a matter of hours.