Consider a consumer that only purchases two goods, X and Y. The government wishes to collect revenue ...

Consider the percentage tax of t on x only. The consumer will select a new optimal bundle (X*,Y*) shown by point B on the graph to the right. The government collects a revenue of  X*t.
Alternatively, the government could impose a tax of t' on both goods. The revenue the government collects is t'I regardless of the bundle chosen.

If t' is set such that the tax revenue is unchanged, then notice that the bundle (X*,Y*) is still just affordable with the tax on both goods. As you can see on the graph, the consumer is better since the budget constraint with the tax on both goods cuts above the IC going through point B.