Generally speaking, this is true for most magnets. When one AXIALLY magnetised magnet is stacked on top of another axially magnetised magnet of the same dimensions and with the poles in the same alignment, the total pull force will increase by approximately 90% compared to the single magnet. As more magnets continue to be added to the stack the increase in magnetic strength will not continue to multiply and will eventually taper off. This fast and simple method of increasing magnetic power can be applied to Neodymium Block shaped magnets and Disc shaped magnets quite easily. You can use this double-up technique for holding applications where the field strength needs to be moderated or when the weight that is supported by the magnets occasionally increases.