>> I am using propser to prepare pdf slides. Now I want to squeeze two slides
>> into one A4 page for print-out. I've tried twoup.sty, it did not shrink
>> the slides before squeezing them into one page.

Hi!
You can use 'pdfpages':

\documentclass[a4paper]{article}
\usepackage{pdfpages}
\begin{document}
\includepdf[nup=1x2,pages=-]{your_slides.pdf}
\end{document}

you can use the class prosper preview (ppr-prv.cls) :  

http://www.ens-lyon.fr/~mgoutell/index.html#soft

I know it can do what you want but I did'nt test

psnup from psutils is fine for this.