Help with subsystems in simulink.

Hi,i was trying to implement a new topology for DC-DC converters in simulink.It basically takes two DC inputs and products various output levels such as summation,difference etc.I am unable to add the simulation file ,hence i will have to explain it.I initially took two voltage sources and created a subsystem for the summation circuit.Now,i want to create a subsystem for the difference circuit too,but with the same input voltage.So basically i want to add the input voltage as inport to the second subsystem.