It depends on what kind of plants they are, actually. At least, that's my opinion. It's always easier to separate them before the root systems are developed. That doesn't mean it would be impossible to separate them later, however.
For many plants, you can just take a large, sturdy knife and cut the root ball in sections, leaving one plant in each section. Then, you would prune back the top growth of each plant and pot it up individually.
I'm guessing, though, that you're referring to the plants you recently purchased ... the Dracaena
and the Sanseveria
. Right? I wouldn't separate the Sanseveria
until spring, and I wouldn't separate the Dracaena
at all. But, of course, there's no rule that says you can't do whatever you like with your own houseplants.
Plants that are going to sprout new growth and become bushier, rather than only growing taller, can be separated, because they'll eventually become nice, shapely plants. The Sanseveria
will do that. The rhizomes will multiply and send up new shoots from the soil, until the plant literally fills the pot.
The only way to get a Dracaena
to sprout new growth from the cane is to cut off the top. Even then, the branches that sprout are usually clustered at the top of the cane. These plants are not propagated by root division. They are propagated by cutting the cane into sections, allowing the cut ends of the sections to callus off (air dry), and then potting them up to create new plants. This is a plant that might not recover from being separated, if the roots are very enmeshed. (Cane plants ... Dracaenas
, bamboo, and others ... are commonly planted in groups of odd numbers, because such groupings are considered to be more pleasing to the eye than single plants or groups of even numbers. That's the theory, anyway.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams