As Balaji already noted, you can Etherchannel any stacked switch with another device, which includes stacked switches. Actually, when doing a port channel between switch stacks, normally you try to use different stack members for redundancy. I.e. one stack member fails, you still have connectivity between the stacks.
BTW, when working with a chassis, the same "logic" often applies by using Etherchannel links on different line cards.
