As far as I know Intel and Motorola never allowd their processors to be 2nd sourced!
8086/8088s were made by at least AMD, Fujitsu, Harris, Mitsubishi, NEC, OKI, and Siemens.
Not all of those were second sources. Some, like NEC, made clones. In NEC's case they used a superior architecture to get added performance in a pin for pin look alike (the original V20 and V30). This lead to a famous long running law suit that NEC eventually "won". "won" means they succeeded in court, but let things drag on so long their potential market was ruined.
Motorola knew from day one that the only way to get into many key applications was to have genuine second sources. They worked with ST, Philips and others to ensure second sources for most of the 68k parts were available on day one. I believe this was more than just exchanging designs. The original 68k was a very early dry plasma etched part, so I think a lot of process exchange was involved.
These days you only need to have a solid second source for very generic parts, like memory. The world has changed.