Can you attach some data?
If the values are correlated, how about if you scatter one versus the other, then fit a line between them? Now if you have the future values of set #1, y1, then you must have the future times, tFuture. So what if you just extrapolate them? You can add some noise if you want.
coefficients = polyfit(y1, y2, 1);
y1Future = y1(tFuture);
y2Future = polyval(coefficients, y1Future);
plot(tFuture, y2Future, 'ms-');
That's just off the top of my head. Can you attach some data so we can see if it seems reasonable?