Hi Danilo,
I've been working on developments for SAP ByDesign only for a couple of years but my understanding is that Solutions that you create run independently of each other and can be activated/deactivated individually. You couldn't normally access custom business objects from solution A in solution B without internal communication.
It would be possible to have the same field name defined in business objects in both solutions and they can both be active at the same time. (Would be confusing, though, if they had the same descriptions as well!)
If both solutions are active then they would both be available for use (as long as they are assigned via Access Rights to relevant users and/or added to Business roles).
Hope that helps!
Maybe someone at SAP might prove me wrong??
Regards
Jay