As far as I know this is not possible with current Modelica Specification. Some tools might support it if you extend MassFlowSource_T:
model MassFlowSource_T_2
extends Modelica.Fluid.Sources.MassFlowSource_T;
// declare m_flow here again with annotation(Evaluate=false);
end MassFlowSource_T_2;
use MassFlowSource_T_2 when you declare source2.
There is some work in progress to extend the way annotations are specified/handled but it will be a while until it makes it into the Modelica Specification: https://trac.modelica.org/Modelica/ticket/1293 (not open to the public yet).