This article covers 

How to offer coupons to multiple subscriptions to a customer at once?

Solution

If your customers have multiple subscriptions, and you wish to offer a coupon of $100 across all subscriptions, you would have to issue a coupon of a certain amount to each of them. 

Please refer here for more details - https://www.chargebee.com/docs/2.0/coupons.html

Since coupons are applied on subscriptions, they are specific to each subscription, and cannot be applied in bulk across subscriptions that belong to the same customer. 

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Applying multiple coupons and manual discounts to a subscription