Hello @henry67852! First, welcome to the class of 2024!!
Here is a link that will give you all the information that you need for transferring from Pratt to Trinity (it is very common) and heard the process isn’t very difficult: https://pratt.duke.edu/undergrad/students/policies/3537
It does appear you are not able to transfer until after your first semester, which sounds accurate as I don’t know anyone who has done it before then. Plus, I think it could be beneficial to get a taste of Pratt before you leave it because it has some advantages. For example, while you are in Pratt, GPA does not matter as much in determining your employment in the future (you will hear this a lot starting in the fall) than doing anything in Trinity (CompSci is one //somewhat// exception but still not at the level as an engineering degree).
The question regarding difficulty is very subject to the type of person you are. Since you got into Pratt, I would guess you are quite STEM-capable. Therefore, I would not be worried about you COMPLETELY bottoming-out when it comes to capability. The same things I said above about difficulty probably sums it up best.
Additionally, majoring in both of them with the concentration is quite the number of classes! So, when selecting your courses for the next few semesters, keep in mind that you don’t have /too/ much time to goof around (there is still some, so don’t stress about this too much). Just be aware of the requirements of both so you don’t find yourself having to overload and take summer sessions later. Luckily, this is one of, if not the most, common double major and the departments know that. Therefore, they have some cool new electives coming (like computational microeconomics) that count towards both the majors!
Also, just a little note on your last question, I think that it is challenging and will definitely require a level and quantity of work that you probably have not experienced but it is doable. In my case, being pre-law, my GPA matters SO much because I am competing for admission with GPAs of humanities and non-STEM majors (their GPAs are just higher on average). So, I have to be incredibly careful about what classes I take and knowing how much I can handle. In finance recruitment, GPAs do matter but not as badly.
I hope that was helpful and let me know if you have any more questions! Whether it be through here or DM! 