No, it’s not possible with this hardware. What you did there in your scenario is took two mono outputs, in which the metronome was essentialy panned right and the synths panned left. But you were using stereo outputs (ie headphones) to capture sound from a mono source so it pans it for you to one ear. The Phones out and the mono speaker line outs send out the same audio, they won’t display nor function as different output channels inside a software.
I assume your metronome is coming from a software application such as a DAW, correct? Certain DAW’s such as Cubase have a control room which is intended to customize headphone mixes for each individual musician.