Unfortunately you have an unrecommended and supported configuration.
At a minnimum you would need to go in to Task Manager and be sure you are completely quit out of Acrobat before opening a PDF.
This isn't a definite fix but seems to work for most.
The only other fix is to remove one of them and repair the other and to not install both on the same Windows machine.
Assuming that you have Reader 9.3.4 already installed on your machine, you can try the following to make Reader 9.3.4 as the default PDF opening application:
1. Right click on any PDF on your system and click on the "Open With" Option in the context menu that appears. In case you get another submenu, click on Chose Program in that menu. (In case you do not see any "Open With" option, then hold down the Shift key and right click on the file)
2. Click on Browse button in the dialog that appears, and select Reader 9. Make sure that you select the checkbox for "Always use the selected program to open this kind of file"
3. Click on OK.
This will make Adobe Reader 9 as the default viewer for all PDFs on your system.
Zowie! This is the answer I was hoping for -- a way to set Adobe Reader 9 as the default for incoming PDFs. Easy, too. Now the test will be to open the next PDF that comes in as an email attachment. I'll let you know how that goes.