I checked with Kelly Castro on the LR team and he said you should be able to save each of your external apps as presets and then they should both be available. He gave me the steps, too:
1. Lightroom > Preferences > External Editing
2. Under Additional External Editor,clci the “Choose” button next to Application
3. Navigate to Applications folder and select Photoshop CS5
4. Set Editing attributes (File Format, Color Space, Bit Depth, Res)
5. Click on preset menu and select “Save Current Settings as New Preset”
6. Name the new Preset “Photoshop CS5” and click “Create”
Thanks! This works, but I would add two comments.
First, one should save their current second editor as a preset (following steps 1, 5 and 6) before creating the CS5 preset.
Second, only two editors can be the primary editors (i.e., only two can be selected by a simple keyboard shortcut, and only two will appear in the preferences window at any one time). However, you can have as many editors as you want as presets: they all appear as options in the menus, and any one of them can be made the secondary editor that can be selected by a keyboard shortcut.