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I lean towards a "force lift" more than a choke. Dooku still has to duke it out with Anakin; I'm not sure he would be able to maintain the concentration to keep the choke going on Obi-Wan (it always took Vader a good 15 seconds minimun) while fighting Anakin. He's either planning to throw Obi-wan or just hold him there and slice Obi-wan with a lightsaber.