Why simultaneously pedalling? In my case It improves direction of my front wheel drive recumbent.
However, it could be used in any bike, may be somebody has another reason for it.

Several instructables explaining how to make a front wheel drive bike are published:

https://www.instructables.com/id/Front-Wheel-Drive-Center-Steer-Semi-Recumbent-Bicy/
https://www.instructables.com/id/Chucks-No-Weld-Recumbent-Bike/
https://www.instructables.com/id/No-Weld-Recumbent-Modifications/
https://www.instructables.com/id/DIY-recumbent-bicycle-made-from-recycled-parts/

These recumbents are easy to do and nice to ride. However, pedalling affects steering because you are pushing with your feet on the sides of the front wheel. To compensate some strength is put your arms as well. Simultaneous pedalling overcomes these problems.

Step 1:

dismount one of the cranks and mount it parallel to the other one. You need a crank puller for that:

https://www.instructables.com/id/How-to-use-a-crank-puller/

Step 2:

Attach springs to pull the crank up to the upper position. Is better with several springs in diferent angular positions, the movement will be smoother.

Of course you may use clipless pedals instead of springs, but be carefull, you risk to fall attached to the pedals if you are not used to them!