This is a question concerning android applications with two different .apks (or two apps contained in the one .apk file)
I have two apps which do completely differen
It depends on your definition of "application". You cannot install 2 applications if you use the more official definition, as you can have only 1 in your manifest.xml
You can define several activities in your manifest.xml, and they can do seperate things, so in that way YOU CAN have 2 things a person might describe as "application" in one APK
Just define multiple activities and use those could be defined as an option, but it depends on your definition of 'application', but in this case I'd say it would work