DStv comes with a satellite signal and Gotv comes with antenna signal, so to connect both cables on one coaxial cable, you’ll need to find a tool that will allows you to send both the satellite and antenna signal to each decoder destination.
The tool you’ll need here is a diplexer, you have to get two diplexers. One diplexer to combine the signal and the other diplexer to split the signal.

Diplexer / signal Combiner
The Video Below Illustration How To Install DStv And Gotv On One Cable: