Certified GKH Gardening Expert
Unless they are "growing" by putting out suckers (stems from the ground), you generally cannot divide them, as the overall majority are best propagated through cuttings. This article will help you:
However, hardy hibiscus can sometimes be divided by cutting the plant in half and is usually done so in spring. Stick a shovel in the middle of the plant between the stalks and cut straight down into the root ball. Then dig out from around the plant (about a foot around). Pop them up out of the ground and then replant.