I've never used Reinsert, but could you pass the same signal out to the two units and bring them back to separate channels to compare levels. Alternately, process a channel and record the result, then swap to the second unit and do the same.
It could just be a case of a tolerance difference in something like a level pot. Many pots are rated +/-10 or 20%, so depending on the QC testing and matching protocols of the manufacturer, differences in levels could be pretty significant.