How do I give my customers a higher discount if they add multiple subscription products to their cart?

If you wish to give your customers a volume discount or tiered discount, when they add a multiple of the subscription products to their cart, or if they add multiple different subscription products to their cart, then for this you can use the active integration we have with the Bundler - Product Bundles app, which allows you to create a Volume discount, which gives a higher discount, the more products they have in their cart. 
The alternative for this would be to create a separate variant for each quantity break on a subscription product. 
For example, for your subscription product, you could have three variants, one for a quantity of 1, one for a quantity of 2, 3, and so on, where each variant would have it's own price.
 

If you wish to use Bundler's Volume discounts like mentioned above, then you can start by installing Bundler - Product Bundles from here: https://apps.shopify.com/bundler-product-bundles
Once you install the app, you can go ahead and click the “Create bundle” button and create a Volume discount.
From there, under the “Volume discounts” section, you can set up the quantity breaks for the volume discounts you want to offer your customers, and under the “Product selection type”, you can select which products you wish to add to the Volume discount.
Last but not least, you have to click the “Save” button to save the Volume discount configuration.
 

If the products you added into the Volume discount are also part of a Subscription rule in Seal Subscriptions, then your customers will also be able to apply the Volume discount to the subscription products in their cart when they checkout.