THX to Claudio Sanches and his Plugin http://wordpress.org/extend/plugins/woocommerce-payment-discounts/
The original Plugin makes it possible to enter a Couponcode and Select all Gateways which should use this Coupon.
I've extended his plugin to fit my needs. Now I can choose a Coupon out of all available Coupons (via Dropdown) for each Gateway seperatly.