Johannesburg is 10 hours ahead of Portland
There is no standard 8-to-6 overlap. Someone joins early or late.
Given Johannesburg is 10 hours ahead of Portland, the realistic answer is that there's no convenient shared slot during normal business hours. Catch the edges of the day: the last hour for Johannesburg or the first hour for Portland is usually the least painful option. Heads up: because Portland changes its clocks and Johannesburg doesn't, the difference moves by an hour twice a year. Confirm the overlap around the DST switch.