That's true, as described in this
document. In your code you're trying to perform the two analog outputs at the same time.
I'm a little rusty with DAQmx, but my understanding is that you should be able to create a task that performs both outputs and both inputs simultaneously with DAQmx 7.3 and above, I think. You can create your tasks in MAX and then in LabVIEW instead of wiring to the "physical channel" input, wire to the "task in" input. If you open up MAX you should see a "NI-DAQmx Tasks" item. Here you can create tasks and name them. So, you can create an analog output task that causes both of your analog outputs to generate a signal. You would want "1 Sample (On Demand)" in the "Task Timing" section. Name this task something like "GenVoltages". In LabVIEW, right-click on the "task in" input of "DAQmx Create Channel". This will create a DAQmx task contant, and when you click on it you should see a list of tasks that have been defined in MAX. Select "GenVoltages". When you want to generate the voltages, use a single instance of "DAQmx Write", select the "1D DBL NChan 1Sampl" instance, and wire in an array instead of a single value. Each element of the array corresponds to a channel in the task. The read part of it should work the same way.