You can install several MIDlets on a phone inside a single jar file if they all belong to the same MIDlet Suite.
You can specify several MIDlets in your jad and manifest according to the MIDP spec:
MIDlet-1: MIDletName1, MIDletIcon1.png, MIDletClass1
MIDlet-2: MIDletName2, MIDletIcon2.png, MIDletClass2
...
Each MIDlet should then be presented individually to the user.
As for hiding a single MIDlet in a MIDlet suite, that would be manufacturer-specifc behavior.
Sony-Ericsson phones allow developers to completely hide an entire MIDlet Suite containing just one MIDlet using jad properties (some of which may only work if your Suite is trusted by the manufacturer) but I've never tried anything beyond that.