Hi Val,
I'd suggest removing the Silicon Labs driver from your computer, then rebooting. Now launch WeatherCat - if it's still showing /dev/cu.SLAB_USBtoUART then there's still a copy somewhere - it might be in a cache, so try one of the many third party cache cleaners. Once you have no /dev/cu.SLAB_USBtoUART showing in WeatherCat, then install the latest one and try again.
If it's still showing two comms ports, it might be worth a mail to SiliconLabs - they may have seen the issue before.