The dollar and RMB amounts on the dual-currency credit card are shared by them. If you spend RMB, then the dollar will be reduced accordingly.

I booked an international flight ticket for my friend with a credit card, and it works normally.

However, it took me only once to know that banks have to charge an extra "foreign exchange settlement channel fee", which is enough.

In addition, it seems that you can't spend dollars in China, so you can use the RMB quota with peace of mind.