Synchronize multiple tasks and multiple boards with X-series USB devices

Question

Hi all,
I am trying to figure out how to synchronize multiple tasks across multiple X-series USB boards (6353, to be precise). 
I have to run a digital output task AND an analog output task on each board.  All four tasks (the DO and AO on Board 1, and the DO and AO on Board 2) must be synchronized.
I know how to synchronize multiple tasks on a single board.  I've been doing that for years.  That works fine.
I know how to synchronize a single task across multiple boards (say, a DO task across 2 different boards).  That works fine too.
For the life of me, I can't figure out how to marry the two.  Everything I've seen, all of the examples, etc. assume you want to do one or the other, or that you have a RTSI cable available.  Are there any examples out there that synchronize multiple tasks across multiple USB boards?
I tried exporting the AO clock signal from my master AO task to PFI12, and then using the signal on PFI12 to as the clock source for the remaining tasks (Dev1/PFI12 physically connected to Dev2/PFI12, and Dev1/PFI12 set as the clock source for Dev1's DO task). 
I have also tried setting Dev1/aoSampClock as the source for Dev1's DO task.
Under both configurations, I invariably get error -89137 when I try to start the tasks -- required resources (PFI12) in use by Dev1/do/SampClock.  I understand what the error is telling me.  What I don't understand is why I'm receiving this error when I don't have any of my tasks configured to use the DO Sample Clock.  It seems as though it is not possible to export a clock signal to a PFI line and simultaneously run a clocked digital task, even if the clock source for that task is specified as something other than the DO Sample Clock?
So, my question is, is it possible to do what I want to do?  And if so, how do I do it?  How do I configure a common clock source across multiple tasks running on multiple USB boards?
Hoping for insight from those wiser than I...
Diane
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Answer 1

Update:  I decided to try using a counter as the source clock.  One cannot use counter 0 to accomplish this, but it does appear to work with counter 1.
I'll take what I can get.  Fortunately I have two spare counters.  (Yes, I'm running counter tasks too -- 6 of them -- but they don't have to be synchronized.)
I have to add both digital and analog input tasks as well -- all synchronous with each other, but not with the output tasks since they'll be running at different sample rates.  I hope I'm able to use a counter as the source clock for those as well.
My head aches.