You are using a blade fuse stamped at 35 amps? Wow, BIG mistake. Swap it for a 20a immediately, it's not ideal but there is no such thing as a 17.5a. A 35 amp blade blows at 70 amps, blade and cartridge fuses are marked differently. Your wiring is not up to a 70 amp fault and you could easily have a fire. The 20a will still leave some room for faults to develop but is a closer match. It may run slightly overheated and you will have to keep an eye on it. Ideally swap back to a glass fusebox, or if you must use blades re-allocate the circuit distribution to a better spread of fuses. That means a lot of maths.
I want to hook up a second fusebox but does anyone know how to join the power to both or best way?
The fuse system in the Mini isn't really bussed, there is a fuse for each different group of circuits. What do you want the new fusebox to supply? Bussed boxes and terminals are available from auto electrical suppliers.