I know this is kinda stupid solution, but 2s timeout is acceptable solution
add_meta_box("addanotheritem", 'Add another', 'addanother_metabox', 'custompost', 'side', 'high');
function addanother_metabox() {
?>
<script type="text/javascript">
function saveAndGo() {
jQuery('button.editor-post-publish-button__button').click();
setTimeout(function() {
window.location='./post-new.php?post_type=custompost';
}, 2000);
return false;
}
</script>
<button onclick='javascript:saveAndGo();'>save & add another</button>
<?php
}