Hello Dmitry,
Today, we don't support combining multiple coupons on a Subscription or an Invoice. You can only have one active coupon on a Subscription which will automatically apply to its invoice(s). When you change the coupon applied, you would also have to handle the time that already passed to calculate the duration of the new coupon if that's the approach you take.
This would let you add some credit to their balance for the number of months you want to give them for free. Each invoice on that subscription will be for the usual amount (say $10), but pull from the customer balance to be paid first until that balance is down to $0. In this case, if you set the balance to -$30, the next 3 invoices would be free and the fourth one would charge them $10.
Hope this helps!
Remi