  The randomness rate of an infinite binary sequence is characterized by the
sequence of ratios between the Kolmogorov complexity and the length of the
initial segments of the sequence. It is known that there is no uniform
effective procedure that transforms one input sequence into another sequence
with higher randomness rate. By contrast, we display such a uniform effective
procedure having as input two independent sequences with positive but
arbitrarily small constant randomness rate. Moreover the transformation is a
truth-table reduction and the output has randomness rate arbitrarily close to
1.
