or do i have to switch my targeting order to make them both die?
Tragic slip checks for morbid the moment it resolves. When each Tragic Slip resolves, check if another creature has died. If one has, slip gives -13/-13. So in your case, if you let your first tragic slip resolve first, and then cast the second tragic slip, both will die. If you put the second slip on the stack before letting the first resolve, the 2/2 will not die.
You seem to be confused about an issue of timing though. When playing instants, you can let them resolve before casting the next instant, or you can choose to cast the next instant in response. In the first case, the first instant resolves and kills the creature, and then you cast and resolve the second instant, which will also kill the creature. However, if you cast the first instant, and then respond with the second instant, it will be on the top of the stack and resolve first.
It's easiest just to let your spells resolve before casting more spells, but if you're ever confused, keep in mind that Tragic Slip or Go for the Throat won't kill their target until they've resolved. If they have not resolved, they have not killed their target yet.