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With multiple devices you open a handle to each one, and then the particular handle you pass to subsequent sub-vis determines which device is talked to.  See "Multiple U3 EFunction Loop Example.vi".  Note that this example has 2 parallel dataflow paths in the loop (one for each U3), so you don't know the order in which the U3s are talked to.  If you want to control the order of execution more, use the error cluster wired in series to control dataflow.