Also, I raised the question at https://retrocomputing.stackexchange.com/q/30743/3722 and one of the answers points out the 1984 Rockwell R65C29 Dual CMOS Microprocessor. It was two standard 6502 on the same die using the same bus to access the same memory... and of course IBM mainframes did it decades before.
Depends on your definition. The 8271 wasn't programmable by anyone but Intel (at least, they never made that a market option), and the second core was more of a bit-oriented coprocessor, sorta like saying the 80486 is a 2-core processor because of the FPU.
