A few things come to mind...
1) Are you certain that the launcher is fully closing after launching the first Slot? If it isn't, then that could be the problem.
2) Are you launching your Character Set or the default game profile? You will want to be launching your Character Set.
3) You could be experiencing the issue that some people do when using the launcher, and, I believe, it's still somewhat of a mystery as to why that happens. Some peoples' computers seem to have an issue with permissions, and so the launcher cannot be used. The workaround would obviously be to choose one of the two game profiles that skip the launcher* (in ISBoxer, upper left pane > select a Character > lower right pane > Game/Game Profile > repeat for all Characters).
* there is currently a bug in IS Build 6383/6392 where the
game profile is missing a required parameter, and you would need to manually add the
-launch parameter to the 64-bit
game profile in Inner Space, under the "Main Executable Parameters" field.