It is NOT recommended but that is not say it can't be done. You are just undertaking it at some risk to the mother plant as well as any offspring. But suckering type shrubs like most hydrangea varieties technically can be divided if done with a great deal of care. When would be in fall just as the plants are going dormant or in late winter/early spring before they fully emerge from dormancy.

Personally, if I wanted to propagate my hydrangeas I would do so from cuttings (about the easiest shrub ever to strike from cuttings) or via layering (easy instructions available online). That way there is absolutely no risk to the plant :-)

Here is a link that might be useful: Dividing shrubs?? Just say NO!

The crown area on most macs is very woody and thick. To cut into this would probably require a hatchet or saws-all. This produces the risk of greatly damaging the crown of the main plant. Then, you'd need to be sure there would be enough canes and roots on the part you hacked off to make another healthy plant, also potentially coming at a cost to the mother plant.

For ones like Nikkos which get very big around at the base, it probably wouldn't be a problem to remove the outermost portions of the crown. But most do not grow that large. In other words, I wouldn't even think of dividing one that wasn't upwards of 10 years old.