OP stands for 'original poster'; i.e. the person who started the thread; you!
Most of the time people use these kinds of plots to look for some correlation; (why else would you combine two different plots in to one?). I favour plotting one versus the other or plotting the two data above each other (two stacked plots). Sometimes there are, however, space-requirements of certain journals that forces you into making these kinds of plots unfortunately.
It is a bit tedious, but I would be pretty happy if the biggest obstacle in my data-analysis is giving a frame-tick specification... The links that Vitaliy posted are kind of what I said (replacing the ticks and rescaling some data) except for the Overlay solution. That one returns an 'Overlay' object, not a Graphics object which might not be handy as you can't use an Overlay in Show. So chose wisely...