how to add top x axis tick?

I have a plotyy plot, and I want add a top x axis which will be the same as the bottom one. I tried to use box on, but this one does not work because I have two different y axes(it appears to be two different ticks on the right side of the y axis). also tried to use xAxisLoation, the problem with this one is that my x axis label also goes to the top which is not I want. Anybody knows how to solve this problem? I will appreciate your help!