It would wash out. Any energy collected would be at the cost of resistance. So add fans to add wind resistance. You could collect energy from coasting and braking, but that’s just tech we’ve been using for years in cars, and it comes at the cost of movement. It actively slows you down because the energy has to come from somewhere. And since energy conversion is hardly one-to-one (loss to heat, etc), storing it into a battery and then pulling it out again means you won’t gain as much as you lose.
Energy cannot be created or destroyed. If you are generating energy, you’re taking it from somewhere, and on a bike, it’s from your forward movement.
Yes, you could collect energy while coasting down a hill, but it would slow you down. Which is fine if you want to slow down; this is the basis for regenerative braking. You might be thinking that a pinwheel spins like crazy in the wind, and that’s just free energy. But a pinwheel doesn’t store anything. To store energy, you need to add resistance, and the more you add, the more energy you collect and the harder it is to spin the wheel.
So at the end of the day, you’ve got a fan at the front of the bike that is either spinning quickly with little resistance and storing little energy or one that is spinning slowly and collecting more. And the slower it spins, the more pushback there is against your forward movement.
Despite there being two batteries, this is still a single system which uses energy to propel the bike forward and collects energy by preventing the bike from moving forward. They offset. The only way to have the energy to propel the bike is by introducing energy from another source (not related to the movement of the bike) such as a battery charged ahead of time or calorie loss of the rider (active pedaling).