Midnite Solar advertises a solar array combiner that allows the installer to cut the DC bus bar to supply up to 3 breakers to (2) separate charge controllers. http://www.affordable-solar.com/store/combiner-boxes-pv-array/Midnite-Solar-MNPV6-Combiner-Box
The combiner uses a common PV (-) for both arrays, which will not work with dual MPPT systems because of the internal current sensing resistors within the MPPT. This creates a low resistance path for stray current to flow. This will corrupt the power output readings and cause erratic operation. See attached drawing. Also attached is a drawing that shows current flow when PV leads are crossed, which will also lead to undesireable operation. When measuring for reversed PV leads, it is necessary to remove PV (+) and PV (-) from BOTH controllers and measure for Voc across the disconnected leads.