Indeed: the first dual core server chips only launched in 2005 afaik with 90nm Denmark/Italy/Egypt Opterons and Paxville Xeons but on the Intel side it wasn't until 2007 when they were in full swing.
first dual core server chips show up generally available in 2001 with IBM POWER4, then HP PA-RISC ones in 2004, and then Opterons which was followed by "emergency" design of essentially two "sockets" on one die of of NetBurst dual core systems.
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.
Ehhhh the MAJC 5200 was generally available in 1999 and I am sure even older designs could be found if we were to dig deep enough. Their market share would also need some digging too.
To quote the announcement: "two VLIW (very long instruction word) microprocessors on a single piece of silicon"
By the time of XVR GPUs Sun was pretty much exiting the workstation market, and unlike Elite3D and Creator3D the competition has widened for "good enough" PC alternatives using Nvidia and similar chips
