Hi Grozeil,
Because Eventide Stompboxes are true stereo devices, they are designed to keep Input 1 and Input 2 discreet. They currently will not sum two inputs to one output (although they will spread one input across two outputs.)
To do what you are describing you would need a y-cable that feeds two 1/4 inch jacks into one 1/4 inch jack, something that looks like this:
http://www.radioshack.com/product/index.jsp?productId=2103842
