Cybex PRIAM Kidboard wheeled board
If the stroller is to be used for another sibling, but the older sibling cannot walk long distances alone, it is time to get the kid board. It can be attached behind the frame in a few simple steps. So you can push your two children at the same time and keep an eye on everything. If the board gets in the way on uneven ground, you can simply remove it and, thanks to its low weight and size, stow it in the lower basket.
The Cybex Kidboard does not have a saddle and can therefore only be used standing. It still leaves enough legroom to push the stroller relaxed and without restrictions.