Order By Case statements

I am having the worst time right now with a Case Statement in my Order By clause. What I need to do is have my query sort by 2 columns when the client is ClientA and have it sort by a different column when it's ClientB.

I tried this:

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ORDERBY(CASEWHENcli.clientname='Client A'THENColumnA,ColumnBEND)ORDERBY(CASEWHENcli.clientname<>'Client A'THENColumnCEND)

So then I tried this because it made sense to me:

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ORDERBYCASEWHENcli.clientname='Client A'THENColumnA,ColumnBELSEColumnCEND

So do any of you possibly know how to make this work?