Raji was not classified by the Packers as "IR with designation to return". He was uncategorically done for the year when he was placed on IR.
Tretter was given that designation and the rule permits only one per team.
When the "designation to return" rule was implemented in 2012, it could only be applied to a guy after the final cut downs. I believe that's still the case. The Packers IRed Raji before that date; they could not have designated him to return at that point even if they wanted to.