Hi
In these sort of examples my answer is always use beforestore or afterstore.
Using beforestore concat values yourself and then have that stored. ie
$someConcatValue = $fields['art_title']->value.'-'.$fields['cat_title']->value;
$config['some_table']['some_column']['some_value'] = $someConcatValue;
or even just something like:
$fields['art_title']->value = $fields['art_title']->value.'-'.$fields['cat_title']->value;
it is up to you how you get the category title rather than id..... many ways to skin a cat....