I see that on most of cryptocurrency exchanges (IDEX, Kucoin, ...), they support pair of coins/tokens from different chains. For example, BTC/ETH, ...
source https://www.google.com/url?rct=j&sa=t&url=https://stackoverflow.com/questions/54015591/how-cryptocurrency-exchange-can-support-pair-of-coins-from-different-chains&ct=ga&cd=CAIyGmM2Nzk3ODA2ZWViNjVjODI6Y29tOmVuOlVT&usg=AFQjCNGrzmuM8c9SQk8o99ZhiAstoT11WA
