Why not name them as you did "sitemap-1" and "sitemap-2" and within you do something like that in a sitemap.php
file:
$number = $_GET['number'];
$entries_per_page = 30000;
for($i = ($number - 1) * $entries_per_page; $i < $number * $entries_per_page; $i++){
// print out the values
}
In addition to that add an entry in your .htaccess like this:
RewriteRule sitemap-([0-9]+).xml$ sitemap.php?number=$1
You can also use the bounries for your SQL query:
$query = "SELECT * FROM table LIMIT ".(($number - 1) * $entries_per_page).", ".$entries_per_page;
I use something similar and in addition to that a sitemap_index.php file looking like this:
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<? for($i = 1; $i <= 6; $i++) : ?>
<sitemap>
<loc><?php echo ROOT_PATH?>sitemap_<?= $i ?>.xml</loc>
<lastmod><?php echo date('Y-m-d')?></lastmod>
</sitemap>
<? endfor ?>
</sitemapindex>
And as most search engines do like sitemaps with a maximum of 1000 entries, you might split your sitemap in even smaller parts. And just use ceil($total_entries/$entries_per_page) for the for loop (replacing the "6").