Of course, mathematics is the first degree. My undergraduate major is information and computing science (mainly mathematics), and my second degree is finance (half of my courses are economics). Mathematics is to be understood, and it depends on the real skill. There are many things to remember in economics, many of which are pushed to mathematics.

Economics can be taught by oneself, while mathematics is taught by teachers. Our teacher often says, "The most capable person studies statistics, but statistics can't study mathematics and mathematics can't study economics". People who are good at math can transfer to economics, physics, finance and so on. However, few people in other majors later turned to mathematics. Many famous economists used to be mathematicians, so you can learn economics as a second degree, but you must learn mathematics well as a first degree.

PS: After I have received more training in mathematics (the first degree), it will be easy to study finance again. It's the same whether it's a double degree or a second degree. I have a second-level degree. If it is a double-degree minor, it may be shallow. If you have a shallow knowledge of mathematics and a deep knowledge of economics, you can't go far.

So it is not the best to study mathematics first, but the first degree must be mathematics. . .