This VST/Buzz plugin is called EZ peacefilter
Ever worked with a crossfader while deejaying? On the left side of the fader you hear record #1, and on the right side record #2.
EZ Peacefilter works almost the same but with frequencies. It needs 2 inputs, and then it will split the two signals, so no overlap is created in the frequency spectrum. Why is this handy? Most of the time when you try to layer 2 'PHAT' sounds with subbass involved, the outcome sucks.
So next to phase-correction-toys, you can use this toy :)
NOTE: you need a VST-host which supports multiple inputs like Buze or REAPER