The holding force is doubled if a magnet is used on both sides

Does the holding force double with 2 magnets?

If you use a magnet on both sides and place them directly on top of each other, the holding force will not be stronger. However, depending on the circumstances/use of the magnets, this may make sense.
However, if you place the 2 magnets on top of each other and use one magnet on one side and have a metal counterpart, it will increase the holding force. Depending on the properties of the magnets, this may even almost double. Stacking magnets is almost as if you were buying the newly achieved size of magnet as one magnet. Only the thin coating of the magnet reduces this minimally.

Briefly explained:
A 10 mm x 5 mm neodymium disc magnet has only minimally (not detectable without a measuring device) higher magnetic holding force than 5 pieces of 10 mm x 1 mm neodymium disc magnets placed on top of each other.

Tip:
If you are not sure about the required holding force of magnets, you can buy them thinner and stack them on top of each other until the holding force is ideal for your project.
 

Magnets have a distance to each other after assembly

It makes sense to use a magnet on both sides. Each magnet has its own magnetic field, which interlocks when there is a gap between them, thus increasing the magnetic holding force
 

How do I achieve the maximum specified holding force?

You will need a polished steel counterpart for this. This counterpart must be at least 1 cm thick and have a larger surface area than the magnet. This is because the magnetic fields of magnets spread in all directions, including to the right and left of the magnet. In order to achieve the maximum holding force, these fields must also be given the opportunity to be as close as possible to the ferromagnetic counterpart.