Sounds like you could use the Advanced Edit’s swap function here.
I’m assuming that all of the notes for “Lead Synth A” are contained in one single track here…
Make a duplicate copy of the track by right-clicking the track in the pattern editor and choosing “Duplicate Track (incl. Patterns)”. You will have now have two tracks containing the same notes.
Open up the Advanced Edit view.
Make sure “Track in song” is enabled (and that the pattern editor cursor is positioned in the duplicate track we just made)
Scroll down to the “Instruments” panel in Advanced Edit.
Set “Src. instr.” to the number of your “Lead Synth A” instrument.
Set “Dest. instr.” to the number of your “Synth B” instrument.
Press the Swap button.
You should now have all of Lead Synth A’s notes mapped to Synth B in its own separate track.
Continue editing as necessary.