Does this device not make it more difficult to walk? That's generally the tradeoff with this type of thing, since the energy has to come from somewhere and all the energy you normally exert moves you forward. At the very least, walking with it probably takes some getting used to.

Depends on the device.

Although it says you wear two devices, I gather from the description that they are independent, so it's not like a dynamo, so there's no additional force of moving in opposition to a magnetic force (like a dynamo) involved.

From the description I'd say that these are completely self contained, that the only extra energy cost will be in carrying the additional weight, and that the reason it suggests wearing two is because the larger part of it's market will have two legs.