Thanks Leo. I figured that was the answer "All LAN ports in each EtherChannel must be the same speed, and you must configure all the ports as either Layer 2 or Layer 3 LAN ports." according to the link provided by Ankur.
We currently have a 2-GB etherchannel between those two stacks. so what I might do is to set up a new 20-GB etherchannel and let STP take care of keeping the 2-GB link in passive mode.