As you are using continuous time, you will need to use a single model with lines that connect the appropriate parts. This is needed so that Simulink can construct the differential equations needed to solve the system over indefinite time.
As soon as you refer to models "talking" to each other, you are talking about discrete events, that one model sometimes sends updates to the other, or one model sometimes queries the state of the other. Discrete events are incompatible with continuous time systems -- unless, that is, you set up portions to be triggered at particular times: if you did that then Simulink would treat those times as boundary conditions and do continuous modeling them.
