rex begonias are rhizomatous, but the distinction is that all begonias in the rex cultorum group originate from crosses with the species rex [B. rex Putzeys] introduced in 1856 from india. this group is noted for their flamboyant colors and patterns, and can be grown similarly to most rhizomatous begonias, except that they may be more sensitive to humidity (in my experience) and mildews.
the rexes can be propagated similarly to any rhizomatous begonia. my favorite method is using the whole leaf african violet style (if you have leaves in abundance), as this will result in a fuller plant much quicker. if you have one or to leaves, it may be best to split them into wedges (2-3 is best, i think, although each vein can potentially turn into a new plant) so you have more chances at success. you can root in water, leaving 2 inches of stem and then transfer to light substrate before the water roots get too long. this method can be iffy, as rot is a factor.

-ming

Thank you ming.