The best way is to use model reference. Add two "Model" blocks, each specified as reference to m1.slx and m2.slx
The other way is just to copy the contents.
new_system()
add_block()
set_param()
Simulink.SubSystem.copyContentsToBlockDiagram(subsys,sys)
Simulink.BlockDiagram.copyContentsToSubsystem(sys,subsys)
save_system()