how to implement two stack pointer in assembler?

how do I implement two stackpointer in assembler?

instead of sub esp, 0xffffffff, one could add esp, 0xffffffff thus overflowing and "implementing" a negative stack. then use two stack pointers, that can borrow from each other and then reset to their stack.

questions is: what assembler flags to use to distinguish between unsigned and signed operations (if one does not use above)