Could you explain why you have to use one object? There are very few applications that do not allow multiple objects be created, each attaching (some mainframes and some apps where hooking is required) - other than that you should always be able to have multiple objects and it is strongly recommended that you do.
If you really do need to use a shared application model my understanding is that you create your 'base' object as usual and then you select that object in the drop down in application modeller wizard.
