You can divide you Phyllostachys at any time - Ideally you want to avoid causing any disturbance whilst shooting as this causes the new shoots to 'abort'. If you need 'loads' of them for a screen hedge, then you will probably be better off by extracting rhizomes and placing them lengthways in a tray. Place the tray in a propagator, and let them sprout. Leave the tray to continue sprouting for a year, until you have a reasonable 'mat' of shoots, and then chop them up into 6 inch squares. Each of these 6 inch squares should give you a new plant, and can probably be divided further on a yearly basis.

If you only require 2 or 3 other plants then continue with 'splitting.' Clean slices are better for the plant and cause less disturbance than 'teasing apart'.

HTH