You are right, on Binance, it is called multi asset mode, when I checked OKX, it was on the trading setting and it is called multi currency margin, is it not referring to coins? Maybe I guessed it wrong.
I was unable to enable it, that made me not to know what it is as it requires someone to have $50000 in his OKX account which I do not have.
I think it's referring to a future contract that includes coins and USD. I'm not so familiar with this multi-currency margin it is new to me compared to the Binance multi-asset mode.
Maybe this link below might help to give you some idea about the multi-currency margin mode on OKX.
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Multi-Currency Margin ModeAbout the required $50k I think it's a future contract price.