Just tested this and it works well.
You will be able to launch each app with a keyboard shortcut of its own by using nothing more than two simple Automator Services, and the Built in keyboard shortcuts.
In this example I am using Preview and Safari.
Create to new Automator Services.
Open Automator and choose 'Service'
First Service.
1,Add a 'Launch Application' action.
2, set the Actions application to 'Preview' by using the drop down in the action.
3, set Service receives to: 'no input' in 'Safari'
(Do this by using the drop downs at the top.)
4, save the Service As Launch Preview
Second Service.
Use the menu file ->New.
And again choose 'Service'
1,Add a 'Launch Application' action.
2, set the Actions application to 'Safari' by using the drop down in the action.
3, set Service receives to: 'no input' in 'Preview'
(Do this by using the drop downs at the top.)
4, save the Service As Launch Safari
System Preferences
Now open System Preferences -> Keyboard.
Go to the Tab:Keyboard Shortcuts
1, Select 'Services' in the left hand pane.
2, In the right hand pane, scroll down to the bottom where 'General' services should be.
You should see your two new services.
3, Double click the blank white space in front of the service name. (As far to the right as the pane allows you )
4,You should now have a text field where you can enter you hotkey for the service.
5,Once done for both service Make sure they are still both ticked and close System Preferences
Now when you are in one or the other app use the keyboard short cuts to switch apps.
Example of keys I used in my test:
While in Safari: crtl+alt+cmd +p t go to Preview
While in Preview: crtl+alt+cmd +s t go to Safari
p.s doing the two services this way will make sure that the hotkeys/keyboard shortcuts, only work when in the said applications. And will not be called when in a third application..