When you set the use_sim_time parameter, the simulation listens to the /clock topic and uses it as the Sim Time. Correct? What node generates this /clock messages? Can I switch them off and publish my own /clock messages instead, so that I can control the Sim Time?

I am asking because I want to connect two simulations so that one would run as normal and the second would use the same /clock signal as the first one. Can I do that?