1} How can I pass coupon directly to the checkout page, without making the user enter it manually?
If you want to pass a discount code to the checkout page URL, you could make use of the following syntax:
Here's a sample URL for reference:
2} How to apply two coupons to the subscription?
Since it is not possible to pass 2 coupons through a hosted page URL, you could create the subscription first and then apply another coupon from your end to the subscription. Note that only two coupons can be applied to a subscription, with each coupon having different Discount Types (Eg - Fixed Amount & Percentage). Here's more information on applying multiple coupons to a subscription.
3} What's the difference between coupons with duration type One Time vs. Limited Period 1 month? How would coupons with duration type 'Limited Period - 1 month' and 'One Time' work for Plans that bills every 6 month?
When a one-time coupon is applied to a subscription at the time of sign-up, discount will be applied only on the charges collected on sign up. Whereas when a limited period 1 month coupon is applied to a subscription at sign-up, discount will be applied on the sign up charges and to any other charge that is added within the one month period.
In these cases, only charges for the first month will have the discount applied. If you add any charge on the second month, the discount will not be applied.
More on Coupons: http://www.chargebee.com/docs/coupons.html