Você também pode usar CROSS APPLY:
SELECT v.Var1, POWER(v.Var1, 2) AS Var2Squared
FROM [Table] t
CROSS APPLY (SELECT t.Column1 + t.Column2 AS Var1) v
;
SELECT v.Var1, POWER(v.Var1, 2) AS Var2Squared
FROM [Table] t
CROSS APPLY (SELECT t.Column1 + t.Column2 AS Var1) v
;