In the Windows MIDI sequencer software
Sekaiju, the Piano Roll window can show multiple tracks at once:
sekaijupianoroll.png. Over on the right side, you can check or uncheck the tracks you want to see. You have to be careful to leave the track you want to edit as the one highlighted on the right side list. (Or selected in the first drop down box in the toolbar.)
However:
Seakiju is only a MIDI editor, it doesn't have any audio or VSTi capability.
The tracks are shown in different colors, but with the same intensity, it doesn't fade out the "background" tracks you aren't currently editing. Also, if you have multiple tracks playing the same pitch, you can't change which track is shown in front, the higher-numbered tracks always get put in front of the lower-numbered tracks.