Plus the additional weight and wind resistance of the bodies (bike + rider position) ;-)
The only viable option is regenerative braking, pretty complex on a pedelec (1) since your engine is not located where you break most (ie. the front wheel) and (2) it'd need to trigger as soon as you stop pedaling, before you activate the mechanical brakes, very uncomfortable.
Tesla Motors does it on the Model S and X, it's super-efficient
as a brake when you released the accelerator and many drivers use it 90% of the time by coasting, except emergency ofc; super rare to change brake pads on a Tesla as a result.
Energy wise though, it only regens 3-5% of the battery max on a full trip and that's world-class hardware/software electrical engineers who shared offices with world-class rocket scientists at SpaceX.
It slowly became a non-critical selling point (not even mentioned on the current prospectus) like the 7-seats configuration.
https://www.tesla.com/models?redirect=no
These folks would probably be better off with a standard frame mount and that second battery they paid for as a back-up on a rack if max range is the objective.