From the error message you posted I'm assuming you are trying to put channels from both boards in the same NI DAQmx Task. That is not possible your the devices you are using, but that does not mean you cannot do what you want.
So, what do you mean by Acquire at the same time?
Do you just want them running the acquisition in parallel? Do you need them starting at the same time? Do you need them fully synchronized (sharing start trigger and sample clock)?
This article can tell you more about this subject.