I'm not so sure this is what you want, but here's an idea:
plot(as.zoo(x), las=1)
par(new=TRUE)
plot(as.zoo(y),
col=2,
bty='n',
xaxt="n",
yaxt="n",
xlab="", ylab="")
axis(4, las=1)
legend("topleft",
legend=c("x","y"),
col=1:2,
lty=1,
cex=0.85)