The company location sets the second currency
Dashboard totals stay in USD. The line under them follows the country or region on the company.
Set it once
In company settings, choose the country or region. That choice picks the local currency under the USD totals. Germany shows EUR. Hong Kong shows HKD. Japan shows JPY. The invoice the client receives does not change.
Where the rate comes from
If the company is in mainland China, the local line uses the rate saved on that invoice. Anywhere else, it uses the day's rate. The local figure is for your books. It is not the amount due.
No second line when the location is already USD
A company in the United States totals in USD only. There is nothing to convert.