First of all, VERY COOL idea. I presently have FSRs on my bed. I'm following this thread with lots of interest.
One thought on redundancy, using your idea of back to back transducers… Maybe set it up such that both sensors are handled separately with signal conditioning, then the outputs are logically "AND"ed together. Then, if both don't trigger as expected (maybe after an RC time constant??), the ANDed output stays triggered (or is somehow latched triggered), which will signal an issue to the Duet's firmware.
Or similarly, have a second output from the signal conditioning board that would change states if the outputs don't behave as expected (i.e. last time, only one triggered). This signal could be monitored by the Duet firmware to identify a fault condition.
In either of these cases, you would probably have to match the gain and offset of the signal conditioning/comparator for each sensor such that they trigger at the same time. Or get really slick about it and let a tiny PIC automatically configure gain/offset for each side and also have it drive the triggered output with a buffered signal.
Hope that all makes sense. I don't know if what I suggest is feasible, but I thought I'd throw it out there.