Johannesburg is 1 hour ahead of Prague
Best overlap: 8:00 AM to 5:00 PM in Prague is 9:00 AM to 6:00 PM in Johannesburg.
Your window to reach both cities is 8:00 AM to 5:00 PM in Prague, which converts to 9:00 AM to 6:00 PM in Johannesburg. 9 hours of shared hours is more than enough — slot meetings wherever they're most convenient for the people involved. Bear in mind Prague shifts for daylight saving while Johannesburg stays fixed — recheck these hours after each clock change.