To be frank, most critics will say two-piece toilets are more prone to breakage than one-piece toilets. I strongly disagree with the intent, not with the principle of this theory. Because durability can never be entirely ensured by the design and type of an established product. In the case of an experimental product, it might be accurate, but not in the case of a renowned two-piece toilet.
You have to understand if the toilet is appropriately designed to provide higher access and ease of use. If so, then one more thing you have to check is the core materials of different accessories.
Why only accessories? Why not the bowl or tank? Wisely asked. Most of the toilets, either one-piece or two-piece, are made of two or three most common core materials, but the accessories like flush knob, toilet seat, flusher position, etc are the sectors where manufacturers will not tell detail to you. These are the sectors you should look to select a durable toilet. The moral is, durability is relative, not type-specific.