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I am not seeing anything about this furnace requiring two returns, although it's not a bad idea based on the size. Keep in mind a furnace tripping the limit has many possible causes including too high gas pressure, undersized supplies and return ducts, a plugged A-coil (for air conditioning), a blower issue, or even just a dirty air filter. I can provide you with an installation manual if you'd like. As far as who installed, if a permit was pulled with the city in which you reside (which is what should have been done) they will have records of the installer.
Here is the installation manual for this unit: