I have two minipages aligned to the top of a frame (I used the option [t] on the frame and the two minipages).

If I had elements in one of the minipage, the other minipage goes down.

I have reproduced a small example (pdf link):



            \textbf{Header}    \\\hline
            \only<2->{ line2\vspace{6cm}\\\hline} 



Do you know how to avoid this behavior?