Thank you JackDevine!
Very nice and simple! Much much better (i.e. more intuitive) than what I had finally managed to dig up:
p.spmap[:a][:yaxis].d[:guide] = "z"
p.spmap[:b][:yaxis].d[:guide] = "y"
Thank you baggepinnen, I initially tried but used the wrong syntax:
plot(x, [y z], layout=l, xlabel="x", ylabel=["z" "y"])
works too!