I'm not sure I understand.
Nobody is stopping you from creating two distinct projects: one B4J desktop and one B4XPages for Android and iOS (of course, I'd say).
If the project is unique, it must work on all three platforms, you can use B4XPages for all 3, with one B4XDrawer.
B4J-B4XPages: you can decide whether to have a single form or more than one.