If you want to store an array in one meta_key, you need to add your different keys in the input name. For my example, this is the retrieve way where you need to be in a foreach loop where $i is increment with $i++ in it. To save value you will need to modify your name input with [‘imageUrl’][] and [‘imagetxt’][].
$imageset = get_post_meta($post_id, 'imageset', true);
$i=0;
foreach($imageset as image){
echo '<input type="hidden" name="imageset['.$i.'][imageUrl]" value="'.$imageset[$i]['imageUrl'].'"/>';
echo '<textarea name="imageset['.$i.'][imageTxt]">'.$imageset[$i]['imageTxt'].'</textarea>';
$i++;
}
I didn’t test it to ajust to your whishes, but I’m sure you’ll get it !