It can be done with Adobe Acrobat.
If you've got a Mac and OSX you can do this in Preview. Make sure you can see the sidebar with the first PDF opened, and then drag the second PDF to the bottom of the last page in the sidebar. Also you can reduce the size of your saved file by selecting (or making) a Quartz filter to 'reduce file size'
Thanks sooooo much. I have a Mac OSX (10.5.8) and Adobe Reader (10.1). I used Preview just like you said and it worked!!! Working on a paper...you saved the day
brilliant person....thanks again.