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The answer is no. The Deletion Condition says that any expression in the generators of a Coxeter group contains a reduced expression as a subexpression. Since $a$ and $b$ don't commute, the unique reduced expression for $ab$ is $ab$, which must therefore occur as a subexpression of any longer expression that evaluates to $ab$, i.e. any such expression must contain an $a$ before a $b$.