One thing that comes to mind is by employing a variation of the Retro Direct concept, you could have both cranks/chains on at the same time, and there wouldn't have to be any mechanical mode switching, just get in the different position. The problem there is it would be a two or one speed unless you found a way to get the freewheels on an internally geared hub. There are some links on that page that show ways of doing Retro Direct with primarily normal parts.

The only way I can think of two do what you describe using a normal rear derailer is have both the front and rear cranks go to turn a spindle with another freewheel on it that is then connected to the rear wheel and derailer. The back cranks would then be turning that spindle with a retro direct type setup to reverse the direction (or maybe there's some other mechanical trick to do so, not sure.)

One thing that comes to mind is by employing a variation of the Retro Direct concept, you could have both cranks/chains on at the same time, and there wouldn't have to be any mechanical mode switching, just get in the different position. The problem there is it would be a two or one speed unless you found a way to get the freewheels on an internally geared hub. There are some links on that page that show ways of doing Retro Direct with primarily normal parts.

The only way I can think of two do what you describe using a normal rear derailer is have both the front and rear cranks go to turn a spindle (idler?) with another freewheel on it that is then connected to the rear wheel and derailer. The back cranks would then be turning that spindle with a retro direct type setup to reverse the direction (or maybe there's some other mechanical trick to do so, not sure.)

One thing that comes to mind is by employing a variation of the Retro Direct concept, you could have both cranks/chains on at the same time, and there wouldn't have to be any mechanical mode switching, just get in the different position. The problem there is it would be a two or one speed unless you found a way to get the freewheels on an internally geared hub. There are some links on that page that show ways of doing it with primarily normal parts.

The only way I can think of two do what you describe using a normal rear derailer is have both the front and rear cranks go to turn a spindle with another freewheel on it that is then connected to the rear wheel and derailer. The back cranks would then be turning that spindle with a retro direct type setup to reverse the direction (or maybe there's some other mechanical trick to do so, not sure.)

One thing that comes to mind is by employing a variation of the Retro Direct concept, you could have both cranks/chains on at the same time, and there wouldn't have to be any mechanical mode switching, just get in the different position. The problem there is it would be a two or one speed unless you found a way to get the freewheels on an internally geared hub. There are some links on that page that show ways of doing Retro Direct with primarily normal parts.

The only way I can think of two do what you describe using a normal rear derailer is have both the front and rear cranks go to turn a spindle with another freewheel on it that is then connected to the rear wheel and derailer. The back cranks would then be turning that spindle with a retro direct type setup to reverse the direction (or maybe there's some other mechanical trick to do so, not sure.)