Dual motor setups have been talked about a bit before, you can dig up the old threads. Generally, the consensus is that there is an overwhelming addition of complexity, weight and rolling resistance for something you can simply achieve by increasing the power of a single motor, and that 15+15 does not = 30 when talking about dual motor setups.

You could definitely try contacting someone about increasing the current to your eZee, http://ebikes.ca has dealt with this motor, for instance. I'm not surprised its top speed is so low, considering the small wheels of the Quando II.