No bro they cut balance once your validity expires. Like in Rajasthan we had to recharge 34 in which some balance+data for 7 days and 6 months sim validity is given and when it expired the balance autocut from account if available. But, BSNL is safe side to keep your secondary sim alive.
Three points to remember : -
- Activate DND
- Turn Off Data services so balance won't deducted.
- Don't expect good offers from company.
- Some plans won't activate by balance they need to be recharged by selecting special recharge option on wallet apps or visiting retailers.