Where are you trying to set this up?

For example Dispatch'r rules work: first applicable is used, rest is ignored. So with two rules you can set up a first "IF". Then in the next one, you know you are in the  "ELSE" situation of the first selection.

I am trying to set up in observer rules.

A ticket flows from 1 group to another group. This group can reject the ticket for insufficient information. so the ticket should be assigned to previous group and previous task