Using one of the cool new features of PHP 5.5, the array_column() function
$collection = array(
array_column($collection, 0),
array_column($collection, 1)
);
EDIT
If you're running a version of PHP less than 5.5, then there's a userland implementation of array_column() here
EDIT #2
For smaller data volumes, but without any PHP version worries, you can also simply transpose the $collection array
$collection = call_user_func_array(
'array_map',
array_merge(
array(NULL),
$collection
)
);
See https://eval.in/84609 for example