Every Blockchain has a separate administration. So, it is not possible to swap tokens on-chain as you would need the first chain to rely on stuff that happens on another chain. To do it when really necessary, you would need a third party that guarantees that the exchange happens correctly.
Neo is unique in that it describes a mechanism in which off chain assets can be exchanged atomically, which is referred to in their whitepaper as "NeoX". Ontology is a collection of protocols of wich part is a implementation of the NeoX concept.