Do you mean just switch between two open applications? or Be able to open the application with a shortcut??
To just switch, use Cmnd-Tab.
Automator with Shortcut:
If you have 10.6, This may be slower but you can use Automator, Very easy:
Step 1. Open Automator, 2. Choose "Services" 3. Set method of input to "no input" and keep other at "all applications" 3. Drag in "Launch Application" from side list. 4. Change the application to what you want 5. Save as something like Open[insert application]
Step 6.Open System Preferences 7. "Keyboard" section and "Keyboard Shortcuts" 8. "Services" 9.Find your new saved service 10. click far left to set you own shortcut!
Problems: Doesn't work if App doesn't support Services.
If it's speed in opening applications, you can use the Apple menu>Recent Items for a recent application. Or figure out how to get the Recent Applications stack on your Dock using Terminal. This is what I use.
Check out Quicksilver, you can set up shortcuts for the applications. Check out Alfred as well, allows shortcuts for apps.