I like to see it the other way around: the partitioning was a smart way of doubling the number of registers that could be expressed in a fixed-length 16-bit ins

I like to see it the other way around: the partitioning was a smart way of doubling the number of registers that could be expressed in a fixed-length 16-bit instruction word. There are many computational instructions that don't make much sense on addresses, especially if you have rich address modes to begin with. And if you'd need to, there was the "movea" instruction for copying between the register files.