The best way would just be to make a normal sized starter and then use it to ferment a small batch of beer (however many gallons of starter you need for your big lager), and then use ALL of the yeast from that batch to ferment your doppelbock. The advantage to this method is you get extra beer out of it, instead of decanting tons of DME juice down the drain. Just keep your "stepper" beer lowish gravity, 1.040-1.050.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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