Hello and welcome to Community Xavier!

Thank you for your post, I have looked into this query for you with in QuickBooks Online you are only able to see both currencies on the invoice when creating the invoice. When the invoice is sent this will only show in the currency set for this customer if they are not using the home currency.

You can request this through the feedback section in the cog wheel of your account or on this link here, any feedback provided goes through to our product development team where they will assess any suggestions and look at implementing them when improving any features or adding new ones.