There is distance restriction on Monthly season tickets - I think it is 150 km. Long back, some of my friends used to have two MST purchased from either end of the journey to an intermediate station. For the journey from Coimbatore to Salem, they will buy two MST, one from cbe to ed and other from slm to ed. But of course carrying more than one MST is an offence under railway rules. I don't know the current position.