how to copy the value of a register into another (Logisim)
If you're aiming for a computer-type system, then what you will have to do is install a 3rd memory unit(this is the RAM in most computers). This memory unit can store the value of the register into one of its vacant spots, copying the contents of the register into said memory. Then, your computer can execute a LOAD instruction to load the byte(or bit group) into your second register.

If your circuit is standalone, you can link the 2 registers with AND gates connected to an ENABLE input. This input will enable copying or not. Then, link the output of said AND gates to the inputs of the second register. This will save the contents of the 1st register to the 2nd register.