To start with, A0 selects the low or high byte ROM for 8-bit read, then A1 - A16 select the individual byte in each ROM out of 65536 possibilities or 64K, so 2 x 64K = 128K bytes of address space. The original ROMs hold 32K bytes each, and they occupy half the address space. If you dump the whole 128K ROM space with "64K ROM", you get 2 images of the ROM, one in the lower half, one in the upper half of the address range. The 68000 normally reads 16 bits at a time, reading a byte from each chip simultaneously. For this reason you want to be sure your code & data sits on even byte boundaries.

Is that any clearer? Original "64K" ROM = 2 x 32K byte (AKA 256K bit) chips = 32K words of 16 bits. Plus ROM = 2 x 64K byte (512K bit) chips.