for i ( 0 .. n ) {
    f( a[i] );  // the required stuff for a[]
    f'( b[i] );  // the required stuff for b[]
    g( a[i], b[i] ); // and whilst we have them, do something to, or accumulate some statistic, that might help the subsequent merge.
}

##</code><code>##

f( a[0] ); f'( b[0] );
for i ( 1 .. n ) {
    f( a[i] );
    f'( a[i] );
    g( a[i], a[i-1] );
    g'( b[i], b[i-1] );
}