x1 = 1:20; % axis in tau scale
x2 = x1.*(128/3); % axis in ms to s /1000
x3 = 1./(x2./1000); % axis in Hz
figure
hL=plot(x2,randn(size(x2)),'r');
hAx=gca;
hAx(2)=axes('Position',hAx(1).Position, ...
'XAxisLocation','top', ...
'Xdir', 'reverse', ...
'YAxisLocation','right', ...
'Color','none');
hL(2)=line(hAx(2),x3,randn(size(x3)),'Color','k');
The key thing you missed is the 'Color','none' so the second axes doesn't occlude the first.
The second requirement is either use the low-level drawing primitives such as line or (you did, just noting) set hold on or plot and the other higher-level routines will still do all their initial house-cleaning and wipe out much of what you just spent all that effort on creating...
