Same thing every time, as soon as the top field is not hidden, the exchange rate calculation goes wrong, hide it, and it works properly.
I'm sorry for the slow response... life happened.
The thing you're trying to do can't be done that way - if you're showing both top and bottom fields, bottom field will always be calculated from the top field. You can't show two unrelated values - bottom field will always be top field multiplied with exchange rate.
What you can do is the following. Hide top field. Use correction 1 and make sure you use currency API (as you already do). That will show GBP exchange rate in bottom field.
Then, under secondary exchange clear exchange URL (as you don't need it). Under "Field 1" (or 2 or 3, whicever you prefer) enter label USD and JSON key "lastPrice".
Then expand your gadget. Bottom field should show GBP exchange rate, and Field 1 should show USD exchange rate. There are two more fields, and you can't hide them unfortunately.
Or simply use two gadgets, one for GBP and one for USD, and show only bottom field on both.